doc: posix: option groups: abbreviate POSIX_C_LANG_SUPPORT

In order to reduce verbosity on the primary Option Groups page,
simply add a note that the POSIX_C_LANG_SUPPORT is considered
supported with newlib, picolibc, or any other libc conforming to
the ISO C standard.

Add a link to the language support page that details C, C++, ..

Signed-off-by: Christopher Friedt <cfriedt@meta.com>