fast-import: clarify documentation of "feature" command

fast-import: clarify documentation of "feature" command

The "feature" command allows streams to specify options for the import
that must not be ignored.  Logically, they are part of the stream,
even though technically most supported features are synonyms to
command-line options.

Make this more obvious by being more explicit about how the analogy
between most "feature" commands and command-line options works.  Treat
the feature (import-marks) that does not fit this analogy separately.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>