doc/gitattributes: clarify "autocrlf overrides eol"

We only override core.eol with core.autocrlf when the latter is set to
something besides "false".  Let's make this more clear, and point the
reader to the git-config definitions, which discuss this in more detail.

Noticed-by: Sergey Lukashev <lukashev.s@ya.ru>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -124,7 +124,9 @@ text file is normalized, its line endings are converted to LF in the
 repository.  To control what line ending style is used in the working
 directory, use the `eol` attribute for a single file and the
 `core.eol` configuration variable for all text files.
-Note that `core.autocrlf` overrides `core.eol`
+Note that setting `core.autocrlf` to `true` or `input` overrides
+`core.eol` (see the definitions of those options in
+linkgit:git-config[1]).
 
 Set::