qapi: Move detection of doc / expression name mismatch

Move the check whether the doc matches the expression name from
check_definition_doc() to check_exprs().  This changes the error
location from the comment to the expression.  Makes sense as the
message talks about the expression: "Definition of '%s' follows
documentation for '%s'".  It's also a step towards getting rid of
check_docs().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-33-git-send-email-armbru@redhat.com>
