docs: Document support for child elements in extend-project

Clarify the existence and behavior of child elements when added to
extend-project.

Change-Id: Id9f270166c8498d4051495b9a1f68360f66e9143
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/553742
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Jeffery Miller <jefferymiller@google.com>
Commit-Queue: Jeffery Miller <jefferymiller@google.com>

docs/manifest-format.md

diff --git a/docs/manifest-format.md b/docs/manifest-format.md
index c3cbe07..f0149dd 100644
--- a/docs/manifest-format.md
+++ b/docs/manifest-format.md
@@ -398,6 +398,11 @@
 existing project definition.  This makes the local manifest more robust
 against changes to the original manifest.
 
+The `extend-project` element can also contain `annotation`, `copyfile`, and
+`linkfile` child elements. These are added to the project's definition. A
+`copyfile` or `linkfile` with a `dest` that already exists in the project
+will overwrite the original.
+
 Attribute `path`: If specified, limit the change to projects checked out
 at the specified path, rather than all projects with the given name.