Merge pull request #339 from ndawe/master

Document RootNumpyUnconvertibleWarning. Fix #332

Author: ndawe

docs/reference/index.rst

@@ -41,6 +41,7 @@ root_numpy
    stretch
    dup_idx
    blockwise_inner_join
+   RootNumpyUnconvertibleWarning
 
 root_numpy.tmva
 ---------------

root_numpy/_warnings.py

@@ -7,6 +7,13 @@
 
 
 class RootNumpyUnconvertibleWarning(RuntimeWarning):
-    pass
+    """
+    This warning is raised when root_numpy is unable to convert a branch into a
+    column of a NumPy array because there is no converter for the type. If the
+    user explicitly requests a branch that cannot be converted, an error is
+    raised. If the user does not specify a list of branches in an attempt to
+    convert all branches, then this warning is raised for each branch that
+    cannot be converted and these branches are merely skipped.
+    """
 
 warnings.simplefilter('always', RootNumpyUnconvertibleWarning)