[2.7] bpo-30754: Document textwrap.dedent blank line behavior. (GH-14469) (GH-14475)

* Added documentation for textwrap.dedent behavior.
(cherry picked from commit eb97b921)
Co-authored-by: tmblweed <tmblweed@users.noreply.github.com>


https://bugs.python.org/issue30754

Doc/library/textwrap.rst

@@ -66,6 +66,9 @@ indentation from strings that have unwanted whitespace to the left of the text.
    of this module incorrectly expanded tabs before searching for common leading
    whitespace.)
 
+   Lines containing only whitespace are ignored in the input and normalized to a
+   single newline character in the output.
+
    For example::
 
       def test():

Lib/textwrap.py

@@ -383,6 +383,8 @@ def dedent(text):
     considered to have no common leading whitespace.  (This behaviour is
     new in Python 2.5; older versions of this module incorrectly
     expanded tabs before searching for common leading whitespace.)
+
+    Entirely blank lines are normalized to a newline character.
     """
     # Look for the longest leading string of spaces and tabs common to
     # all lines.