Update gaia.rst

Updated the documentation to account the possibility to query the early Data Release 3.

The exact wording and the position of the example may be changed if deemed necessary, but the example should be working correctly.

Author: Juma7C9

docs/gaia/gaia.rst

@@ -77,6 +77,13 @@ Examples
 
 This query searches for all the objects contained in an arbitrary rectangular projection of the sky.
 
+It is possible to choose which data release to query, by default the Gaia DR2 catalogue is used. For example:
+
+.. code-block:: python
+
+  >>> Gaia.MAIN_GAIA_TABLE = "gaiadr2.gaia_source"  # Select Data Release 2, default
+  >>> Gaia.MAIN_GAIA_TABLE = "gaiaedr3.gaia_source" # Select early Data Release 3
+
 The following example searches for all the sources contained in an squared region of side = 0.1
 degrees around an specific point in RA/Dec coordinates.