gitattributes: -crlf is not binary

The description of crlf attribute incorrectly said that "-crlf" means
binary.  It is true that for binary files you would want "-crlf", but
that is not the same thing.

We also have supported attribute macros and via that mechanism a handy
"binary" to specify "-crlf -diff" at the same time.  It was not documented
anywhere as far as I can tell, even though the support was there from
the very beginning.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -105,9 +105,8 @@ Set::
 
 Unset::
 
-	Unsetting the `crlf` attribute on a path is meant to
-	mark the path as a "binary" file.  The path never goes
-	through line endings conversion upon checkin/checkout.
+	Unsetting the `crlf` attribute on a path tells git not to
+	attempt any end-of-line conversion upon checkin or checkout.
 
 Unspecified::
 
@@ -482,6 +481,41 @@ in the file.  E.g. the string `$Format:%H$` will be replaced by the
 commit hash.
 
 
+USING ATTRIBUTE MACROS
+----------------------
+
+You do not want any end-of-line conversions applied to, nor textual diffs
+produced for, any binary file you track.  You would need to specify e.g.
+
+------------
+*.jpg -crlf -diff
+------------
+
+but that may become cumbersome, when you have many attributes.  Using
+attribute macros, you can specify groups of attributes set or unset at
+the same time.  The system knows a built-in attribute macro, `binary`:
+
+------------
+*.jpg binary
+------------
+
+which is equivalent to the above.  Note that the attribute macros can only
+be "Set" (see the above example that sets "binary" macro as if it were an
+ordinary attribute --- setting it in turn unsets "crlf" and "diff").
+
+
+DEFINING ATTRIBUTE MACROS
+-------------------------
+
+Custom attribute macros can be defined only in the `.gitattributes` file
+at the toplevel (i.e. not in any subdirectory).  The built-in attribute
+macro "binary" is equivalent to:
+
+------------
+[attr]binary -diff -crlf
+------------
+
+
 EXAMPLE
 -------