qapi: Improve error message for description following section

The error message is bad when the section is untagged.  For instance,
test case doc-interleaved-section produces "'@foobar:' can't follow
'Note' section", which is okay, but if we drop the "Note:" tag, we get
"'@foobar:' can't follow 'None' section, which is bad.

Change the error message to "description of '@foobar:' follows a
section".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230510141637.3685080-1-armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
[Conflict with commit 3e32dca3 resolved]