Document when cherry pick fields in ChangeInfo are set

These fields are only set if the cherry-pick was done through the Gerrit
REST API, but not if the cherry-pick was done in a local Git client and
then pushed to Gerrit.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I0a073f1abf77995ed48e8c18f2344c4139570304

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index f035514..16d5ad5 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -6488,8 +6488,12 @@
 The callers must not rely on the format of the submission ID.
 |`cherry_pick_of_change`   |optional|
 The numeric Change-Id of the change that this change was cherry-picked from.
+Only set if the cherry-pick has been done through the Gerrit REST API (and
+not if a cherry-picked commit was pushed).
 |`cherry_pick_of_patch_set`|optional|
 The patchset number of the change that this change was cherry-picked from.
+Only set if the cherry-pick has been done through the Gerrit REST API (and
+not if a cherry-picked commit was pushed).
 |`contains_git_conflicts`  |optional, not set if `false`|
 Whether the change contains conflicts. +
 If `true`, some of the file contents of the change contain git conflict