docs: note the type of core.attributesfile

docs: note the type of core.attributesfile

The previous wording:

> Path expansions are made the same way as for `core.excludesFile`.

required one to check the docs for 'core.excludesFile' and from there
the definition of the pathname variable type to understand the path
expansion behaviour of this variable. Instead, just link directly to the
pathname type.

This change is basically the same rewording as was done to
'core.excludesFile' in dca83abd (config: describe 'pathname' value
type, 2016-04-29).

Signed-off-by: Matthew Hughes <matthewhughes934@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>