manifest-format.txt: Add documentation for GetManifest RPC method

Add documentation of the GetManifest RPC method in the
manifest-server section.

Change-Id: I5cda5929bc8a0ca9d3f2b9da63216427041d2823
diff --git a/docs/manifest-format.txt b/docs/manifest-format.txt
index db48668..338e021 100644
--- a/docs/manifest-format.txt
+++ b/docs/manifest-format.txt
@@ -125,14 +125,15 @@
 At most one manifest-server may be specified. The url attribute
 is used to specify the URL of a manifest server, which is an
-XML RPC service that will return a manifest in which each project
-is pegged to a known good revision for the current branch and
+XML RPC service.
-The manifest server should implement:
+The manifest server should implement the following RPC methods:
   GetApprovedManifest(branch, target)
+Return a manifest in which each project is pegged to a known good revision
+for the current branch and target.
 The target to use is defined by environment variables TARGET_PRODUCT
 and TARGET_BUILD_VARIANT. These variables are used to create a string
 of the form $TARGET_PRODUCT-$TARGET_BUILD_VARIANT, e.g. passion-userdebug.
@@ -140,6 +141,11 @@
 GetApprovedManifest without the target parameter and the manifest server
 should choose a reasonable default target.
+  GetManifest(tag)
+Return a manifest in which each project is pegged to the revision at
+the specified tag.
 Element project