Doc: Adds example of joining a series to a dataframe.

Resolves: pandas-dev#12550

Author: Patrick D. Park

doc/source/merging.rst

@@ -152,7 +152,7 @@ functionality below.
 Set logic on the other axes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-When gluing together multiple DataFrames, you have a choice of how to handle
+When gluing together multiple ``DataFrame``s, you have a choice of how to handle 
 the other axes (other than the one being concatenated). This can be done in 
 the following three ways:
 
@@ -323,13 +323,6 @@ 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
@@ -583,7 +576,7 @@ and ``right`` is a subclass of DataFrame, the return type will still be
 
 ``merge`` is a function in the pandas namespace, and it is also available as a
 ``DataFrame`` instance method :meth:`~DataFrame.merge`, with the calling 
-``DataFrame`` being implicitly considered the left object in the join.
+``DataFrame `` being implicitly considered the left object in the join.
 
 The related :meth:`~DataFrame.join` method, uses ``merge`` internally for the
 index-on-index (by default) and column(s)-on-index join. If you are joining on
@@ -636,7 +629,7 @@ key combination:
 
 Here is a more complicated example with multiple join keys. Only the keys 
 appearing in ``left`` and ``right`` are present (the intersection), since 
-``how='inner'`` by default.
+``how='inner'```by default.
 
 .. ipython:: python
 
@@ -721,7 +714,32 @@ either the left or right tables, the values in the joined table will be
           labels=['left', 'right'], vertical=False);
    plt.close('all');
 
-Here is another example with duplicate join keys in DataFrames:
+To join a Series and a DataFrame, the Series has to be transformed into a DataFrame first:
+
+.. ipython:: python
+
+   df = pd.DataFrame({"Let": ["A", "B", "C"], "Num": [1, 2, 3]})
+   df
+
+   # The series has a multi-index with levels corresponding to columns in the DataFrame we want to merge with
+   ser = pd.Series(
+       ['a', 'b', 'c', 'd', 'e', 'f'],
+       index=pd.MultiIndex.from_arrays([["A", "B", "C"]*2, [1, 2, 3, 4, 5, 6]])
+       )
+   ser
+
+   # Name the row index levels
+   ser.index.names=['Let','Num']
+   ser
+
+   # reset_index turns the multi-level row index into columns, which requires a DataFrame
+   df2 = ser.reset_index()
+   type(df2)
+
+   # Now we merge the DataFrames
+   pd.merge(df, df2, on=['Let','Num'])
+
+Here is another example with duplicate join keys in ``DataFrame``s:
 
 .. ipython:: python
 
@@ -1202,7 +1220,7 @@ Overlapping value columns
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The merge ``suffixes`` argument takes a tuple of list of strings to append to
-overlapping column names in the input ``DataFrame``\ s to disambiguate the result
+overlapping column names in the input ``DataFrame``s to disambiguate the result
 columns:
 
 .. ipython:: python