DOC: added a reference to DataFrame assign in concatenate section of merging

Because of various stack overflow answers we are directed to the merging
page where a link to DataFrame.assign would be better. Adding the
reference here will make people's code better. At least it would have
for me.

Author: obilodeau

doc/source/merging.rst

@@ -323,6 +323,13 @@ the name of the ``Series``.
           labels=['df1', 's1'], vertical=False);
    plt.close('all');
 
+.. note::
+
+   Since we're concatenating a ``Series`` to a ``DataFrame``, we could have
+   achieved the same result with :meth:`DataFrame.assign`. To concatenate an
+   arbitrary number of pandas objects (``DataFrame`` or ``Series``), use
+   ``concat``.
+
 If unnamed ``Series`` are passed they will be numbered consecutively.
 
 .. ipython:: python

doc/source/whatsnew/v0.23.0.txt

@@ -658,6 +658,7 @@ Documentation Changes
   Rewrote some sentences for greater clarity, added more dynamic references
   to functions, methods and classes.
   (:issue:`18941`, :issue:`18948`, :issue:`18973`, :issue:`19017`)
+- Added a reference to :func:`DataFrame.assign` in the concatenate section of the merging documentation (:issue:`18665`)
 -
 
 .. _whatsnew_0230.bug_fixes: