qapi: Avoid unwanted blank lines in QAPIDoc

qapi: Avoid unwanted blank lines in QAPIDoc

We silently fix missing #optional tags for QAPIDoc by appending a line
"#optional" to the section's .content.  However, this interferes with
.__repr__ stripping trailing blank lines from .content.

Use new ArgSection instance variable .optional instead, and leave
.content alone.

To permit testing .optional in texi_body(), clean up texi_enum()'s
hack to add empty documentation for undocumented enum values: add an
ArgSection instead of ''.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Message-Id: <1489582656[email protected]>