DOC: Fixes LaTeX build error

The PDF doc build was chocking on the `Parameters` section being the
first item.

Author: Tom Augspurger <tom.augspurger88@gmail.com>

Closes #12586 from TomAugspurger/doc-build-error and squashes the following commits:

6843139 [Tom Augspurger] DOC: Fixes LaTeX build error

Author: TomAugspurger

pandas/tseries/resample.py

@@ -414,6 +414,8 @@ def backfill(self, limit=None):
 
     def fillna(self, method, limit=None):
         """
+        Fill missing values
+
         Parameters
         ----------
         method : str, method of resampling ('ffill', 'bfill')