gitattributes: document more clearly where macros are allowed

The old text made it sound like macros are only allowed in the
.gitattributes file at the top-level of the working tree.  Make it
clear that they are also allowed in $GIT_DIR/info/attributes and in
the global and system-wide gitattributes files.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -930,9 +930,12 @@ state.
 DEFINING MACRO ATTRIBUTES
 -------------------------
 
-Custom macro attributes can be defined only in the `.gitattributes`
+Custom macro attributes can be defined only in top-level gitattributes
-file at the toplevel (i.e. not in any subdirectory).  The built-in
+files (`$GIT_DIR/info/attributes`, the `.gitattributes` file at the
-macro attribute "binary" is equivalent to:
+top level of the working tree, or the global or system-wide
+gitattributes files), not in `.gitattributes` files in working tree
+subdirectories.  The built-in macro attribute "binary" is equivalent
+to:
 
 ------------
 [attr]binary -diff -merge -text