pwcache.3: Explicitly document OOM condition

The pwcache functions allocate memory, and may return NULL pointers if that
allocation fails and the corresponding uid or gid was not found in the local
password database.  Document this behavior.

Sponsored by:	Dell EMC Isilon

contrib/libc-pwcache/pwcache.3

@@ -56,7 +56,7 @@
 .\"
 .\"     @(#)pwcache.3	8.1 (Berkeley) 6/9/93
 .\"
-.Dd October 19, 2012
+.Dd May 5, 2020
 .Dt PWCACHE 3
 .Os
 .Sh NAME
@@ -193,6 +193,15 @@ and
 may be
 .Dv NULL
 pointers.
+.Sh ERRORS
+If insufficient memory is available,
+.Fn user_from_uid
+and
+.Fn group_from_gid
+may return NULL pointers.
+.Va errno
+is set to
+.Er ENOMEM .
 .Sh SEE ALSO
 .Xr getgrgid 3 ,
 .Xr getgrnam 3 ,