doc: posix: overview: clarification on conformant implementations

Any C library with conformant implementations of POSIX features,
options, or option groups can be used in place of the versions
provided with Zephyr's POSIX implementation as long as there is
no source licensing compatibility or linking clause violation.

Implementations of such features, options, or option groups can
be used in whole or in part, as long as there is no conflict
with the rest of Zephyr or the application.

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