Clarify reference counting of uvm_map.

share/man/man9/uvm_map.9

@@ -1,4 +1,4 @@
-.\"	$NetBSD: uvm_map.9,v 1.10 2017/05/20 08:00:47 wiz Exp $
+.\"	$NetBSD: uvm_map.9,v 1.11 2018/08/20 14:29:05 riastradh Exp $
 .\"
 .\" Copyright (c) 1998 Matthew R. Green
 .\" All rights reserved.
@@ -136,11 +136,21 @@ is any other value, then a regular mapping is performed at this offset.
 The start address of the map will be returned in
 .Fa startp .
 .El
-Note that
+If
+.Fa uobj
+is supplied, then
 .Fn uvm_map
-does not add a reference to
-.Fa uobj ;
-it is the caller's responsibility to do so.
+.Em consumes
+the caller's reference to
+.Fa uobj
+on success;
+.Fn uvm_unmap
+will release it when removing this mapping.
+On failure,
+.Fn uvm_map
+leaves the reference count of
+.Fa uobj
+unmodified.
 .Pp
 .Fa align
 specifies alignment of mapping unless