--- a/taskcluster/docs/parameters.rst
+++ b/taskcluster/docs/parameters.rst
@@ -17,16 +17,20 @@ file if necessary, rather than starting 
 complete set of parameters.
 
 The properties of the parameters object are described here, divided rougly by
 topic.
 
 Push Information
 ----------------
 
+``triggered_by``
+   The event that precipitated this decision task; one of ``"nightly"`` or
+   ``"push"``.
+
 ``base_repository``
    The repository from which to do an initial clone, utilizing any available
    caching.
 
 ``head_repository``
    The repository containing the changeset to be built.  This may differ from
    ``base_repository`` in cases where ``base_repository`` is likely to be cached
    and only a few additional commits are needed from ``head_repository``.