gh-107959: clarify Unix-availability of `os.lchmod()` (GH-107960)

gh-107959: clarify Unix-availability of `os.lchmod()` (GH-107960)

POSIX specifies that implementations are not required to support changing the
file mode of symbolic links, but may do so.
Consequently, `lchmod()` is not part of POSIX (but mentioned for implementations
which do support the above).

The current wording of the availability of `os.lchmod()` is rather vague and
improved to clearly tell which POSIX/Unix/BSD-like support the function in
general (those that support changing the file mode of symbolic links).
Further, some examples of major implementations are added.

Data for the BSDs taken from their online manpages.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>