ls-tree documentation: enhance notes on subdirectory and pathspec behaviour

When run in a working copy subdirectory, git-ls-tree will automagically
add the prefix to the pathspec, which can result in an unexpected behavior
when the tree object accessed is not the root tree object.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-ls-tree.txt

@@ -16,10 +16,20 @@ SYNOPSIS
 DESCRIPTION
 -----------
 Lists the contents of a given tree object, like what "/bin/ls -a" does
-in the current working directory. Note that the usage is subtly different,
-though - 'paths' denote just a list of patterns to match, e.g. so specifying
-directory name (without '-r') will behave differently, and order of the
-arguments does not matter.
+in the current working directory.  Note that:
+
+ - the behaviour is slightly different from that of "/bin/ls" in that the
+   'paths' denote just a list of patterns to match, e.g. so specifying
+   directory name (without '-r') will behave differently, and order of the
+   arguments does not matter.
+
+ - the behaviour is similar to that of "/bin/ls" in that the 'paths' is
+   taken as relative to the current working directory.  E.g. when you are
+   in a directory 'sub' that has a directory 'dir', you can run 'git
+   ls-tree -r HEAD dir' to list the contents of the tree (that is
+   'sub/dir' in 'HEAD').  You don't want to give a tree that is not at the
+   root level (e.g. 'git ls-tree -r HEAD:sub dir') in this case, as that
+   would result in asking for 'sub/sub/dir' in the 'HEAD' commit.
 
 OPTIONS
 -------