docs: correct documentation about eol attribute

docs: correct documentation about eol attribute

The documentation for the eol attribute states that it is "effectively
setting the text attribute".  However, this implies that it forces the
text attribute to always be set, which has not been the case since
6523728499 ("convert: unify the "auto" handling of CRLF", 2016-06-28).
Let's avoid confusing users (and the present author when trying to
describe Git's behavior to others) by clearly documenting in which
cases the "eol" attribute has effect.

Specifically, the attribute always has an effect unless the file is
explicitly set as -text, or the file is set as text=auto and the file is
detected as binary.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>