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: NJonathan Nieder <jrnieder@gmail.com>
Acked-by: NSverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -878,28 +878,25 @@ Require that fast-import supports the specified feature, or abort if
 it does not.
 
 ....
-	'feature' SP <feature> LF
+	'feature' SP <feature> ('=' <argument>)? LF
 ....
 
-The <feature> part of the command may be any string matching
-^[a-zA-Z][a-zA-Z-]*$ and should be understood by fast-import.
-
-Feature work identical as their option counterparts with the
-exception of the import-marks feature, see below.
-
-The following features are currently supported:
-
-* date-format
-* import-marks
-* export-marks
-* relative-marks
-* no-relative-marks
-* force
-
-The import-marks behaves differently from when it is specified as
-commandline option in that only one "feature import-marks" is allowed
-per stream. Also, any --import-marks= specified on the commandline
-will override those from the stream (if any).
+The <feature> part of the command may be any one of the following:
+
+date-format::
+export-marks::
+relative-marks::
+no-relative-marks::
+force::
+	Act as though the corresponding command-line option with
+	a leading '--' was passed on the command line
+	(see OPTIONS, above).
+
+import-marks::
+	Like --import-marks except in two respects: first, only one
+	"feature import-marks" command is allowed per stream;
+	second, an --import-marks= command-line option overrides
+	any "feature import-marks" command in the stream.
 
 `option`
 ~~~~~~~~