Add documentation to public fields of UCred struct

Author: Joe Ellis

library/std/src/sys/unix/ext/ucred.rs

@@ -12,9 +12,16 @@ use libc::{gid_t, pid_t, uid_t};
 #[unstable(feature = "peer_credentials_unix_socket", issue = "42839", reason = "unstable")]
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct UCred {
+    /// The UID part of the peer credential. This is the effective UID of the process at the domain
+    /// socket's endpoint.
     pub uid: uid_t,
+    /// The GID part of the peer credential. This is the effective GID of the process at the domain
+    /// socket's endpoint.
     pub gid: gid_t,
-    // pid field is an option because it is not supported on some platforms.
+    /// The PID part of the peer credential. This field is optional because the PID part of the
+    /// peer credentials is not supported on every platform. On platforms where the mechanism to
+    /// discover the PID exists, this field will be populated to the PID of the process at the
+    /// domain socket's endpoint. Otherwise, it will be set to None.
     pub pid: Option<pid_t>,
 }