DOC: adding SIA1 note to IRSA docs

Author: bsipocz

docs/ipac/irsa/irsa.rst

@@ -299,6 +299,13 @@ Simple image access queries
 Image Access VO service. In the following example we are looking for Spitzer
 Enhanced Imaging products in the centre of the COSMOS field as a `~astropy.table.Table`.
 
+.. note::
+   There are two versions of SIA queries. This IRSA module in astroquery supports the newer,
+   version 2. However not all IRSA image collections have been migrated into
+   the newer protocol yet, if you want access to these, please use
+   `PyVO <https://pyvo.readthedocs.io/en/latest/>`_ directly as showcased in these
+   `IRSA tutorials <https://caltech-ipac.github.io/irsa-tutorials/#accessing-irsa-s-on-premises-holdings-using-vo-protocols>`__.
+
 .. doctest-remote-data::
 
    >>> from astroquery.ipac.irsa import Irsa