no bug - run-on-project: Add a new note r=Callek

Differential Revision: https://phabricator.services.mozilla.com/D76730

taskcluster/docs/attributes.rst

@@ -61,6 +61,14 @@ anywhere, unless its build platform is specified explicitly in try syntax.
     to define that a task will run on `try`.
 
 
+.. note::
+
+    A given task `[taskA]` may not respect `run-on-projects` if there another task
+    `[taskB]` which is scheduled to run (such as via run-on-projects) which depends it
+    `[taskA]`. Because by nature of `TaskB` running we must run `TaskA`.
+
+    See `bug 1640603 <https://bugzilla.mozilla.org/show_bug.cgi?id=1640603#c5>`_ as example.
+
 run_on_hg_branches
 ==================