Be explicit about legacy var additions/omissions

Behavior changes between v2 environment variables and the v3
zuul_legavy_vars filter may cause regressions in some translated
jobs, so be very explicit about which variables were intentionally
omitted, which variables are completely new, and which variables are
now appearing in pipelines they did not previously.

Change-Id: Idf032f9903b21e383a733ab9ff1a6ec80f636e4a
diff --git a/zuul/ansible/filter/zuul_filters.py b/zuul/ansible/filter/zuul_filters.py
index 4092c06..fa21f6b 100644
--- a/zuul/ansible/filter/zuul_filters.py
+++ b/zuul/ansible/filter/zuul_filters.py
@@ -14,10 +14,19 @@
 
 
 def zuul_legacy_vars(zuul):
-    # omitted:
-    # ZUUL_URL
-    # ZUUL_REF
+    # intentionally omitted:
+    # BASE_LOG_PATH
+    # JOB_TAGS
+    # LOG_PATH
     # ZUUL_COMMIT
+    # ZUUL_REF
+    # ZUUL_URL
+    #
+    # newly added to all builds:
+    # ZUUL_SHORT_PROJECT_NAME
+    #
+    # existing in most builds but newly added for periodic:
+    # ZUUL_BRANCH
 
     short_name = zuul['project']['name'].split('/')[-1]
     params = dict(ZUUL_UUID=zuul['build'],