Correctly document the return value of strerror() and strerror_r() and the contents of the returned buffer for unknown error codes.
Correctly document the return value of strerror() and strerror_r() and
the contents of the returned buffer for unknown error codes.

PR:		docs/72578
Submitted by:	Jilles Tjoelker <jilles@stack.nl>
MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase