Add /usr/lib/debug directory to hier(7)

The canonical standalone debug directory established by the GNU
toolchain is /usr/lib/debug, and we use it when WITH_DEBUG_FILES is set.
Mention it in the file system hierarchy page.

Reviewed by:	bcr
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D1134

share/man/man7/hier.7

@@ -28,7 +28,7 @@
 .\"	@(#)hier.7	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd July 25, 2014
+.Dd November 10, 2014
 .Dt HIER 7
 .Os
 .Sh NAME
@@ -380,6 +380,8 @@ shared libraries for compatibility
 .It Pa aout/
 a.out backward compatibility libraries
 .El
+.It Pa debug/
+standalone debug data for the base system libraries and binaries
 .It Pa dtrace/
 DTrace library scripts
 .It Pa engines/