mount.2: Clarify term "unprivileged mount namespace" in Keno's patch

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/mount.2

@@ -651,7 +651,10 @@ or
 An attempt was made to bind mount an unbindable mount.
 .TP
 .BR EINVAL
-In an unprivileged mount namespace, a bind mount operation
+In an unprivileged mount namespace
+(i.e., a mount namespace owned by a user namespace
+that was created by an unprivileged user),
+a bind mount operation
 .RB ( MS_BIND )
 was attempted without specifying
 .RB ( MS_REC ),