core.autocrlf documentation: mention the crlf attribute

The description of the configuration variable is obsolete and
wrong (saying only file content is used), not just incomplete.
It has used the attribute mechanism for a long time.

The documentation of gitattributes mentions the core.autocrlf
configuration variable in its description of crlf attribute.
Refer to the gitattributes documentation from here as well.

Signed-off-by: Matthew Ogilvie <mmogilvi_git@miniinfo.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -169,9 +169,10 @@ core.autocrlf::
 	writing to the filesystem.  The variable can be set to
 	'input', in which case the conversion happens only while
 	reading from the filesystem but files are written out with
-	`LF` at the end of lines.  Currently, which paths to consider
-	"text" (i.e. be subjected to the autocrlf mechanism) is
-	decided purely based on the contents.
+	`LF` at the end of lines.  A file is considered
+	"text" (i.e. be subjected to the autocrlf mechanism) based on
+	the file's `crlf` attribute, or if `crlf` is unspecified,
+	based on the file's contents.  See linkgit:gitattributes[5].
 
 core.safecrlf::
 	If true, makes git check if converting `CRLF` as controlled by