Clarify how the cap_get_pid() argument is interpreted.

Addresses:

  https://bugzilla.kernel.org/show_bug.cgi?id=215812

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>
diff --git a/doc/cap_get_proc.3 b/doc/cap_get_proc.3
index 496c06e..7c3bc81 100644
--- a/doc/cap_get_proc.3
+++ b/doc/cap_get_proc.3
@@ -76,7 +76,11 @@
 is 0, then the calling process's capabilities are returned.)
 This information can also be obtained from the
 .I /proc/<pid>/status
-file.
+file. Note, when the caller is operating within a
+.RB ( CLONE_NEWPID )
+namespace, the numerical
+.I pid
+argument is interpreted in the range of that namespace.
 .PP
 .BR cap_get_bound ()
 with a
@@ -392,5 +396,6 @@
 .BR cap_from_text (3),
 .BR cap_get_file (3),
 .BR cap_init (3),
+.BR namespaces (7),
 .BR psx_syscall (3),
 .BR capabilities (7).