git-remote-helpers.txt: document missing capabilities

Specifically, document the 'export' and '(im|ex)port-marks'
capabilities as well as the export command, which were
undocumented (but in active use).

Signed-off-by: Max Horn <max@quendi.de>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-remote-helpers.txt

@@ -106,6 +106,10 @@ to the `capabilities` command (see COMMANDS, below).
 	For listing remote refs and fetching the associated history to
 	the local object store.
 
+'export'::
+	For listing remote refs and pushing specified objects from a
+	fast-import stream to remote refs.
+
 'import'::
 	For listing remote refs and fetching the associated history as
 	a fast-import stream.
@@ -143,6 +147,16 @@ there is an implied `refspec *:*`.
 	This is to prevent mixing commands and fast-import responses on the
 	helper's stdin.
 
+'export-marks' <file>::
+	This modifies the 'export' capability, instructing git to dump the
+	internal marks table to <file> when complete. For details,
+	read up on '--export-marks=<file>' in linkgit:git-fast-export[1].
+
+'import-marks' <file>::
+	This modifies the 'export' capability, instructing git to load the
+	marks specified in <file> before processing any input. For details,
+	read up on '--import-marks=<file>' in linkgit:git-fast-export[1].
+
 Capabilities for Pushing
 ~~~~~~~~~~~~~~~~~~~~~~~~
 'connect'::
@@ -158,9 +172,18 @@ Supported commands: 'connect'.
 +
 Supported commands: 'list for-push', 'push'.
 
-If a helper advertises both 'connect' and 'push', git will use
-'connect' if possible and fall back to 'push' if the helper requests
-so when connecting (see the 'connect' command under COMMANDS).
+'export'::
+	Can discover remote refs and push specified objects from a
+	fast-import stream to remote refs.
++
+Supported commands: 'list for-push', 'export'.
+
+If a helper advertises 'connect', git will use it if possible and
+fall back to another capability if the helper requests so when
+connecting (see the 'connect' command under COMMANDS).
+When choosing between 'push' and 'export', git prefers 'push'.
+Other frontends may have some other order of preference.
+
 
 Capabilities for Fetching
 ~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -307,6 +330,22 @@ stdin.
 +
 Supported if the helper has the 'import' capability.
 
+'export'::
+	Instructs the remote helper that any subsequent input is
+	part of a fast-import stream (generated by 'git fast-export')
+	containing objects which should be pushed to the remote.
++
+Especially useful for interoperability with a foreign versioning
+system.
++
+The 'export-marks' and 'import-marks' capabilities, if specified,
+affect this command in so far as they are passed on to 'git
+fast-export', which then will load/store a table of marks for
+local objects. This can be used to implement for incremental
+operations.
++
+Supported if the helper has the 'export' capability.
+
 'connect' <service>::
 	Connects to given service. Standard input and standard output
 	of helper are connected to specified service (git prefix is