doc: Improve some grammar and sentences

90fbf7c has taken care of that for HEAD.  This includes the portion of
the fixes that applies to the documentation, where needed depending on
the branch.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20201227202604.GC26311@telsasoft.com
Backpatch-through: 9.5

doc/src/sgml/catalogs.sgml

@@ -4466,7 +4466,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        inherited columns are to be arranged.  The count starts at 1.
       </para>
       <para>
-       Indexes can not have multiple inheritance, since they can only inherit
+       Indexes cannot have multiple inheritance, since they can only inherit
        when using declarative partitioning.
       </para></entry>
      </row>

doc/src/sgml/pgstatstatements.sgml

@@ -358,7 +358,7 @@
        <structfield>wal_bytes</structfield> <type>numeric</type>
       </para>
       <para>
-       Total amount of WAL bytes generated by the statement
+       Total amount of WAL generated by the statement in bytes
       </para></entry>
      </row>
     </tbody>

doc/src/sgml/ref/explain.sgml

@@ -198,9 +198,9 @@ ROLLBACK;
     <listitem>
      <para>
       Include information on WAL record generation. Specifically, include the
-      number of records, number of full page images (fpi) and amount of WAL
-      bytes generated.  In text format, only non-zero values are printed.  This
-      parameter may only be used when <literal>ANALYZE</literal> is also
+      number of records, number of full page images (fpi) and the amount of WAL
+      generated in bytes. In text format, only non-zero values are printed.
+      This parameter may only be used when <literal>ANALYZE</literal> is also
       enabled.  It defaults to <literal>FALSE</literal>.
      </para>
     </listitem>

doc/src/sgml/ref/pg_dump.sgml

@@ -630,7 +630,7 @@ PostgreSQL documentation
       <listitem>
        <para>
         Specify the compression level to use.  Zero means no compression.
-        For the custom archive format, this specifies compression of
+        For the custom and directory archive formats, this specifies compression of
         individual table-data segments, and the default is to compress
         at a moderate level.
         For plain text output, setting a nonzero compression level causes

doc/src/sgml/ref/pg_verifybackup.sgml

@@ -40,7 +40,7 @@ PostgreSQL documentation
 
   <para>
    It is important to note that the validation which is performed by
-   <application>pg_verifybackup</application> does not and can not include
+   <application>pg_verifybackup</application> does not and cannot include
    every check which will be performed by a running server when attempting
    to make use of the backup. Even if you use this tool, you should still
    perform test restores and verify that the resulting databases work as

doc/src/sgml/sources.sgml

@@ -103,7 +103,7 @@ less -x4
     message text.  In addition there are optional elements, the most
     common of which is an error identifier code that follows the SQL spec's
     SQLSTATE conventions.
-    <function>ereport</function> itself is just a shell macro, that exists
+    <function>ereport</function> itself is just a shell macro that exists
     mainly for the syntactic convenience of making message generation
     look like a single function call in the C source code.  The only parameter
     accepted directly by <function>ereport</function> is the severity level.

doc/src/sgml/wal.sgml

@@ -580,7 +580,7 @@
    Independently of <varname>max_wal_size</varname>,
    the most recent <xref linkend="guc-wal-keep-size"/> megabytes of
    WAL files plus one additional WAL file are
-   kept at all times. Also, if WAL archiving is used, old segments can not be
+   kept at all times. Also, if WAL archiving is used, old segments cannot be
    removed or recycled until they are archived. If WAL archiving cannot keep up
    with the pace that WAL is generated, or if <varname>archive_command</varname>
    fails repeatedly, old WAL files will accumulate in <filename>pg_wal</filename>