docs/devel/qapi-code-gen: Reorder sections for readability

docs/devel/qapi-code-gen: Reorder sections for readability

Section "QMP/Guest agent schema" starts with a brief introduction,
then subsection "Comments", then subsection "Schema overview" (more
elaborate introduction), and only then talks about schema entities
like types, commands, and so forth.

Subsection "Comments" is long and tiring: almost 500 words, mostly
about doc comments.  Move the doc comment part to its own subsection
"Documentation comments" at the very end of "QMP/Guest agent schema".

Subsection "Schema overview" explains naming rules at considerable
length: 250 words.  Move this part to its own subsection "Naming rules
and reserved names" right after the subsections on schema entities.

Subsection "Enumeration types" is wedged between "Struct types" and
"Union types".  Move it before "Struct types".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-13-armbru@redhat.com>