doc/gitattributes: mention non-recursive behavior

The gitattributes documentation claims that the pattern
rules are largely the same as for gitignore. However, the
rules for recursion are different.

In an ideal world, we would make them the same (if for
nothing else than consistency and simplicity), but that
would create backwards compatibility issues. For some
discussion, see this thread:

  https://public-inbox.org/git/slrnkldd3g.1l4.jan@majutsushi.net/

But let's at least document the differences instead of
actively misleading the user by claiming that they're the
same.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -56,9 +56,16 @@ Unspecified::
 
 When more than one pattern matches the path, a later line
 overrides an earlier line.  This overriding is done per
-attribute.  The rules how the pattern matches paths are the
-same as in `.gitignore` files; see linkgit:gitignore[5].
-Unlike `.gitignore`, negative patterns are forbidden.
+attribute.
+
+The rules by which the pattern matches paths are the same as in
+`.gitignore` files (see linkgit:gitignore[5]), with a few exceptions:
+
+  - negative patterns are forbidden
+
+  - patterns that match a directory do not recursively match paths
+    inside that directory (so using the trailing-slash `path/` syntax is
+    pointless in an attributes file; use `path/**` instead)
 
 When deciding what attributes are assigned to a path, Git
 consults `$GIT_DIR/info/attributes` file (which has the highest