manifest_xml: - Added doc and testing of unknown tags/elements.

Added this test to verify that older versions of repo can handle
"<superproject" element. Tested by adding "<iankaz" unknown element.

Tested the code with the following commands.

$ ./run_tests tests/test_manifest_xml.py
$ ./run_tests -v

Bug: https://crbug.com/gerrit/13709
Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: I858d56f38cefcfcd14474efdd631a5a940c3ce47
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/293482
Reviewed-by: Mike Frysinger <vapier@google.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/docs/manifest-format.md b/docs/manifest-format.md
index 8e5e2874..6156333a 100644
--- a/docs/manifest-format.md
+++ b/docs/manifest-format.md
@@ -109,6 +109,12 @@ following DTD:
 ]>
 ```
 
+For compatibility purposes across repo releases, all unknown elements are
+silently ignored.  However, repo reserves all possible names for itself for
+future use.  If you want to use custom elements, the `x-*` namespace is
+reserved for that purpose, and repo guarantees to never allocate any
+corresponding names.
+
 A description of the elements and their attributes follows.