doc: posix: options: make column size consistent

For some reason, when the "Supported" coumn of the posix option
documentation included large strings, it would distort the size
of the entire table.

Move blurbs about undefined behaviour and failing with ENOSYS
to a paragraph above the table, so that all tables have a
homogeneous size.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>