Move various doc/introspection comments from `gthread-posix.c` (which is
platform-specific) to `gthread.c` (which is not). Having the
introspection annotations and doc comments in a platform-independent
file means that they are seen by the build process on all platforms, and
we don’t end up with unintrospectable APIs on some platforms, or
platform-specific annotation differences.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3399