[libc][docs] Generate docs for signal.h & optimized is_implemented func (#88028)

Fixes #87835

This patch added the documentation for the POSIX functions according to
[n3096](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf)
Section 7.14, and gives the *docgen.py* script a more elegant *is_implemented*
function.