Documentation/fast-import: clarify summary for feature command

In most cases, "feature <foo>" does not just require that the feature
exists, but also changes the behavior by enabling it.

Cases where the feature is only requested like cat-blob, notes or ls are
clearly documented below.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -361,8 +361,8 @@ and control the current import process.  More detailed discussion
 	`--cat-blob-fd` or `stdout` if unspecified.
 
 `feature`::
-	Require that fast-import supports the specified feature, or
-	abort if it does not.
+	Enable the specified feature. This requires that fast-import
+	supports the specified feature, and aborts if it does not.
 
 `option`::
 	Specify any of the options listed under OPTIONS that do not