qapidoc: introduce QAPISchemaIfCond.docgen()

qapidoc: introduce QAPISchemaIfCond.docgen()

Instead of building the condition documentation from a list of string,
use the result generated from QAPISchemaIfCond.docgen().

This changes the generated documentation from:
- COND1, COND2... (where COND1, COND2 are Literal nodes, and ',' is Text)
to:
- COND1 and COND2 (the whole string as a Literal node)

This will allow us to generate more complex conditions in the following
patches, such as "(COND1 and COND2) or COND3".

Adding back the differentiated formatting is left to the wish list.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210804083105.97531-6-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[TODO comment added]
Signed-off-by: Markus Armbruster <armbru@redhat.com>