Add ``RETURN VALUES'' and ``ERRORS'' sections since getpgid(2) can fail.  Also fix a minor grammar nit (it's -> its).
Add ``RETURN VALUES'' and ``ERRORS'' sections since getpgid(2) can
fail.  Also fix a minor grammar nit (it's -> its).

PR:		26520

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