Add `RETURN VALUES'' and `ERRORS'' sections since getpgid(2) can

fail.  Also fix a minor grammar nit (it's -> its).

PR:		26520

lib/libc/sys/getpgrp.2

@@ -76,6 +76,23 @@ and
 .Fn tcsetpgrp
 calls
 are used to get/set the process group of the control terminal.
+.Sh RETURN VALUES
+The
+.Fn getpgrp
+call always succeeds.
+Upon successful completion, the
+.Fn getpgid
+call returns the process group of the specified process;
+otherwise, it returns a value of \-1 and sets
+.Va errno
+to indicate the error.
+.Sh ERRORS
+.Fn getpgid
+will succeed unless:
+.Bl -tag -width Er
+.It Bq Er ESRCH
+there is no process whose process ID equals fo
+.Fa pid
 .Sh SEE ALSO
 .Xr getsid 2 ,
 .Xr setpgid 2 ,
@@ -87,7 +104,7 @@ function call appeared in
 .Bx 4.0 .
 The
 .Fn getpgid
-function call is derived from it's usage in System V Release 4.
+function call is derived from its usage in System V Release 4.
 .Sh STANDARDS
 The
 .Fn getpgrp