Doc: document bpchar, clarify relationship of text and varchar.

For some reason the "bpchar" type name was defined nowhere in
our SGML docs, although several places refer to it in passing.
Give it a proper mention under Character Types.

While here, also provide an explanation of how the text and varchar
types relate.  The previous wording seemed to be doing its best
to sweep text under the rug, which doesn't seem very appropriate
given its prominence in other parts of the docs.

Minor rearrangements and word-smithing for clarity, too.

Laurenz Albe and Tom Lane, per gripe from Yanliang Lei

Discussion: https://postgr.es/m/120b3084.56b6.1833b5ffe4b.Coremail.msdnchina@163.com

doc/src/sgml/datatype.sgml

@@ -1154,6 +1154,10 @@ SELECT '52093.89'::money::numeric::float8;
     <primary>varchar</primary>
    </indexterm>
 
+   <indexterm zone="datatype-character">
+    <primary>bpchar</primary>
+   </indexterm>
+
     <table id="datatype-character-table">
      <title>Character Types</title>
      <tgroup cols="2">
@@ -1169,7 +1173,7 @@ SELECT '52093.89'::money::numeric::float8;
         <entry>variable-length with limit</entry>
        </row>
        <row>
-        <entry><type>character(<replaceable>n</replaceable>)</type>, <type>char(<replaceable>n</replaceable>)</type></entry>
+        <entry><type>character(<replaceable>n</replaceable>)</type>, <type>char(<replaceable>n</replaceable>)</type>, <type>bpchar(<replaceable>n</replaceable>)</type></entry>
         <entry>fixed-length, blank padded</entry>
        </row>
        <row>
@@ -1196,7 +1200,14 @@ SELECT '52093.89'::money::numeric::float8;
     error, unless the excess characters are all spaces, in which case
     the string will be truncated to the maximum length. (This somewhat
     bizarre exception is required by the <acronym>SQL</acronym>
-    standard.) If the string to be stored is shorter than the declared
+    standard.)
+    However, if one explicitly casts a value to <type>character
+    varying(<replaceable>n</replaceable>)</type> or
+    <type>character(<replaceable>n</replaceable>)</type>, then an over-length
+    value will be truncated to <replaceable>n</replaceable> characters without
+    raising an error. (This too is required by the
+    <acronym>SQL</acronym> standard.)
+    If the string to be stored is shorter than the declared
     length, values of type <type>character</type> will be space-padded;
     values of type <type>character varying</type> will simply store the
     shorter
@@ -1204,33 +1215,35 @@ SELECT '52093.89'::money::numeric::float8;
    </para>
 
    <para>
-    If one explicitly casts a value to <type>character
+    In addition, <productname>PostgreSQL</productname> provides the
-    varying(<replaceable>n</replaceable>)</type> or
+    <type>text</type> type, which stores strings of any length.
-    <type>character(<replaceable>n</replaceable>)</type>, then an over-length
+    Although the <type>text</type> type is not in the
-    value will be truncated to <replaceable>n</replaceable> characters without
+    <acronym>SQL</acronym> standard, several other SQL database
-    raising an error. (This too is required by the
+    management systems have it as well.
-    <acronym>SQL</acronym> standard.)
+    <type>text</type> is <productname>PostgreSQL</productname>'s native
+    string data type, in that most built-in functions operating on strings
+    are declared to take or return <type>text</type> not <type>character
+    varying</type>.  For many purposes, <type>character varying</type>
+    acts as though it were a <link linkend="domains">domain</link>
+    over <type>text</type>.
    </para>
 
    <para>
-    The notations <type>varchar(<replaceable>n</replaceable>)</type> and
+    The type name <type>varchar</type> is an alias for <type>character
-    <type>char(<replaceable>n</replaceable>)</type> are aliases for <type>character
+    varying</type>, while <type>char</type> and <type>bpchar</type> are
-    varying(<replaceable>n</replaceable>)</type> and
+    aliases for <type>character</type>.
-    <type>character(<replaceable>n</replaceable>)</type>, respectively.
+    The <type>varchar</type> and <type>char</type> aliases are defined in
-    If specified, the length must be greater than zero and cannot exceed
+    the <acronym>SQL</acronym> standard, but <type>bpchar</type> is
-    10485760.
+    a <productname>PostgreSQL</productname> extension.
+   </para>
+
+   <para>
+    If specified, the length <replaceable>n</replaceable> must be greater
+    than zero and cannot exceed 10485760.
     <type>character</type> without length specifier is equivalent to
     <type>character(1)</type>. If <type>character varying</type> is used
     without length specifier, the type accepts strings of any size. The
-    latter is a <productname>PostgreSQL</productname> extension.
+    latter behavior is a <productname>PostgreSQL</productname> extension.
-   </para>
-
-   <para>
-    In addition, <productname>PostgreSQL</productname> provides the
-    <type>text</type> type, which stores strings of any length.
-    Although the type <type>text</type> is not in the
-    <acronym>SQL</acronym> standard, several other SQL database
-    management systems have it as well.
    </para>
 
    <para>