Add links as well for the other optional databases

Author: kwankyu

src/sage/databases/cremona.py

@@ -9,10 +9,11 @@
 is included by default with Sage.  It contains Weierstrass equations,
 rank, and torsion for curves up to conductor 10000.
 
-The large database includes all curves in John Cremona's tables. It
-also includes data related to the BSD conjecture and modular degrees
-for all of these curves, and generators for the Mordell-Weil
-groups. To install it, run the following in the shell::
+The large database includes all curves in John Cremona's tables. It also
+includes data related to the BSD conjecture and modular degrees for all of
+these curves, and generators for the Mordell-Weil groups. To install it via the
+optional :ref:`database_cremona_ellcurve <spkg_database_cremona_ellcurve>`
+package, run the following command in the shell ::
 
     sage -i database_cremona_ellcurve
 

src/sage/databases/stein_watkins.py

@@ -1,18 +1,16 @@
 r"""
 The Stein-Watkins table of elliptic curves
 
-Sage gives access to the Stein-Watkins table of elliptic curves, via an
-optional package that you must install. This is a huge database of elliptic
-curves. You can install the database (a 2.6GB package) with the command
-
-::
+Sage gives access to the Stein-Watkins table of elliptic curves, via the
+optional :ref:`database_stein_watkins <spkg_database_stein_watkins>` package
+that you must install. This is a huge database of elliptic curves. You can
+install the database (a 2.6GB package) with the command ::
 
     sage -i database_stein_watkins
 
 You can also automatically download a small version, which takes much less
-time, using the command
-
-::
+time, via the optional :ref:`database_stein_watkins <spkg_database_stein_watkins_mini>`
+package using the command ::
 
     sage -i database_stein_watkins_mini