docs/devel/qapi-code-gen: Rewrite compatibility considerations

docs/devel/qapi-code-gen: Rewrite compatibility considerations

We have some compatibility advice buried in sections "Enumeration
types" and "Struct types".  Compatibility is actually about commands
and events.  It devolves to the types used there.  All kinds of types,
not just enumerations and structs.

Replace the existing advice by a new section "Compatibility
considerations".

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-14-armbru@redhat.com>
[Squash in paragraph on invisible schema changes, as per Eric's review]