qobject: Correct JSON lexer grammar comments

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>