Add missing manifest format documentation

Documentation of the "sync-j", "sync-c", "sync-s" and "upstream"
attributes is missing/incomplete.  Add it.

Change-Id: I74977f819f603c520ef3818f85c3b51399cd2b94

docs/manifest-format.txt

diff --git a/docs/manifest-format.txt b/docs/manifest-format.txt
index f6dba64..0bf09f6 100644
--- a/docs/manifest-format.txt
+++ b/docs/manifest-format.txt
@@ -55,6 +55,7 @@
     <!ATTLIST project groups   CDATA #IMPLIED>
     <!ATTLIST project sync-c   CDATA #IMPLIED>
     <!ATTLIST project sync-s   CDATA #IMPLIED>
+    <!ATTLIST project upstream CDATA #IMPLIED>
 
     <!ELEMENT annotation (EMPTY)>
     <!ATTLIST annotation name  CDATA #REQUIRED>
@@ -122,6 +123,15 @@
 `refs/heads/master`).  Project elements lacking their own
 revision attribute will use this revision.
 
+Attribute `sync_j`: Number of parallel jobs to use when synching.
+
+Attribute `sync_c`: Set to true to only sync the given Git
+branch (specified in the `revision` attribute) rather than the
+whole ref space.  Project elements lacking a sync_c element of
+their own will use this value.
+
+Attribute `sync_s`: Set to true to also sync sub-projects.
+
 
 Element manifest-server
 -----------------------
@@ -202,6 +212,16 @@
 If the project has a parent element, the `name` and `path` here
 are the prefixed ones.
 
+Attribute `sync_c`: Set to true to only sync the given Git
+branch (specified in the `revision` attribute) rather than the
+whole ref space.
+
+Attribute `sync_s`: Set to true to also sync sub-projects.
+
+Attribute `upstream`: Name of the Git branch in which a sha1
+can be found.  Used when syncing a revision locked manifest in
+-c mode to avoid having to sync the entire ref space.
+
 Element annotation
 ------------------