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
6523728 ("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>

Documentation/gitattributes.txt

@@ -160,11 +160,12 @@ unspecified.
 ^^^^^
 
 This attribute sets a specific line-ending style to be used in the
-working directory.  It enables end-of-line conversion without any
-content checks, effectively setting the `text` attribute.  Note that
-setting this attribute on paths which are in the index with CRLF line
-endings may make the paths to be considered dirty.  Adding the path to
-the index again will normalize the line endings in the index.
+working directory.  This attribute has effect only if the `text`
+attribute is set or unspecified, or if it is set to `auto` and the file
+is detected as text.  Note that setting this attribute on paths which
+are in the index with CRLF line endings may make the paths to be
+considered dirty. Adding the path to the index again will normalize the
+line endings in the index.
 
 Set to string value "crlf"::