DOC: document pandas.types.common

Closes pandas-devgh-15895.

Author: gfyoung

pandas/types/common.py

@@ -31,6 +31,19 @@
 
 
 def _ensure_float(arr):
+    """
+    Ensure that an array object has a float dtype if possible.
+
+    Parameters
+    ----------
+    arr : The array whose data type we want to enforce as float.
+
+    Returns
+    -------
+    float_arr : The original array cast to the float dtype if
+                possible. Otherwise, the original array is returned.
+    """
+
     if issubclass(arr.dtype.type, (np.integer, np.bool_)):
         arr = arr.astype(float)
     return arr
@@ -46,6 +59,19 @@ def _ensure_float(arr):
 
 
 def _ensure_categorical(arr):
+    """
+    Ensure that an array object is a Categorical (if not already).
+
+    Parameters
+    ----------
+    arr : The array that we want to convert into a Categorical.
+
+    Returns
+    -------
+    cat_arr : The original array cast as a Categorical. If it already
+              is a Categorical, we return as is.
+    """
+
     if not is_categorical(arr):
         from pandas import Categorical
         arr = Categorical(arr)