initgroups.3: Clarify that ENOMEM is a possible value for 'errno'

The current phrasing could be interpreted as meaning that initgroups(3)
could return ENOMEM instead of -1, which it actually did until I fixed
it in commit 0b018cfd81 ("initgroups(3): Fix return value on
allocation failure").

While here, mention setgroups(2) as a system call rather than a library
function.

MFC with:       0b018cfd81 ("initgroups(3): Fix return value on allocation failure")
Sponsored by:   The FreeBSD Foundation

lib/libc/gen/initgroups.3

@@ -33,7 +33,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 17, 2025
+.Dd October 9, 2025
 .Dt INITGROUPS 3
 .Os
 .Sh NAME
@@ -67,9 +67,9 @@ The
 .Fn initgroups
 function may fail and set
 .Va errno
-to any of the errors specified for the library function
-.Xr setgroups 2 .
-It may also return:
+to any of the errors specified for the
+.Xr setgroups 2
+system call, or to:
 .Bl -tag -width Er
 .It Bq Er ENOMEM
 The