Add documentation for the 'bidi-import' capability of remote-helpers

Signed-off-by: Florian Achleitner <florian.achleitner.2.6.31@gmail.com>
Acked-by: David Michael Barr <b@rr-dav.id.au>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-remote-helpers.txt

@@ -98,6 +98,20 @@ advertised with this capability must cover all refs reported by
 the list command.  If no 'refspec' capability is advertised,
 there is an implied `refspec *:*`.
 
+'bidi-import'::
+	The fast-import commands 'cat-blob' and 'ls' can be used by remote-helpers
+	to retrieve information about blobs and trees that already exist in
+	fast-import's memory. This requires a channel from fast-import to the
+	remote-helper.
+	If it is advertised in addition to "import", git establishes a pipe from
+	fast-import to the remote-helper's stdin.
+	It follows that git and fast-import are both connected to the
+	remote-helper's stdin. Because git can send multiple commands to
+	the remote-helper it is required that helpers that use 'bidi-import'
+	buffer all 'import' commands of a batch before sending data to fast-import.
+	This is to prevent mixing commands and fast-import responses on the
+	helper's stdin.
+
 Capabilities for Pushing
 ~~~~~~~~~~~~~~~~~~~~~~~~
 'connect'::
@@ -286,7 +300,12 @@ terminated with a blank line. For each batch of 'import', the remote
 helper should produce a fast-import stream terminated by a 'done'
 command.
 +
-Supported if the helper has the "import" capability.
+Note that if the 'bidi-import' capability is used the complete batch
+sequence has to be buffered before starting to send data to fast-import
+to prevent mixing of commands and fast-import responses on the helper's
+stdin.
++
+Supported if the helper has the 'import' capability.
 
 'connect' <service>::
 	Connects to given service. Standard input and standard output