qobject: Correct JSON lexer grammar comments

Fix the regex comments describing what we parse as JSON.  No change
to the lexer itself, just to the comments:
- The "" and '' string construction was missing alternation between
different escape sequences
- The construction for numbers forgot to handle optional leading '-'
- The construction for numbers was grouped incorrectly so that it
didn't permit '0.1'
- The construction for numbers forgot to mark the exponent as optional
- No mention that our '' string and "\'" are JSON extensions
- No mention of our %d and related extensions when constructing JSON

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1465526889-8339-2-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Eric's regexp simplification squashed in]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qobject/json-lexer.c

@@ -18,11 +18,20 @@
 #define MAX_TOKEN_SIZE (64ULL << 20)
 
 /*
- * \"([^\\\"]|(\\\"\\'\\\\\\/\\b\\f\\n\\r\\t\\u[0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F]))*\"
- * '([^\\']|(\\\"\\'\\\\\\/\\b\\f\\n\\r\\t\\u[0-9a-fA-F][0-9a-fA-F][0-9a-fA-F][0-9a-fA-F]))*'
- * 0|([1-9][0-9]*(.[0-9]+)?([eE]([-+])?[0-9]+))
+ * Required by JSON (RFC 7159):
+ *
+ * \"([^\\\"]|\\[\"'\\/bfnrt]|\\u[0-9a-fA-F]{4})*\"
+ * -?(0|[1-9][0-9]*)(.[0-9]+)?([eE][-+]?[0-9]+)?
  * [{}\[\],:]
- * [a-z]+
+ * [a-z]+   # covers null, true, false
+ *
+ * Extension of '' strings:
+ *
+ * '([^\\']|\\[\"'\\/bfnrt]|\\u[0-9a-fA-F]{4})*'
+ *
+ * Extension for vararg handling in JSON construction:
+ *
+ * %((l|ll|I64)?d|[ipsf])
  *
  */