gh-101845: pyspecific: Fix i18n for availability directive (GH-101846)

pyspecific: Fix i18n for availability directive

If the directive has content, the previous code would nest paragraph
nodes from that content inside a general paragraph node, which confuses
Sphinx and leads it to drop the content when translating. Instead, use a
container node for the body.

Also use set_source_info so that any warnings have location info.