DOC: adding note about cached query

Author: bsipocz

docs/ipac/irsa/irsa.rst

@@ -388,6 +388,10 @@ only collections with a given search string in the collection names.
 The ``servicetype`` argument is used to filter for image collections, using ``'SIA'``,
 or spectral collections (also see below), using ``'SSA'``.
 
+.. note:: The query underneath ``list_collections`` is cached on the server
+          side, and therefore should return quickly with results.
+          If you experience query timeout, please open an IRSA helpdesk ticket.
+
 .. doctest-remote-data::
 
    >>> from astroquery.ipac.irsa import Irsa