bpo-30754: Document textwrap.dedent blank line behavior. (GH-14469)

* Added documentation for textwrap.dedent behavior.
* Remove an obsolete note about pre-2.5 behavior from the docstring.

Doc/library/textwrap.rst

@@ -77,6 +77,9 @@ functions should be good enough; otherwise, you should use an instance of
    equal: the lines ``"  hello"`` and ``"\thello"`` are considered to have no
    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

@@ -420,9 +420,9 @@ def dedent(text):
 
     Note that tabs and spaces are both treated as whitespace, but they
     are not equal: the lines "  hello" and "\\thello" are
-    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.)
+    considered to have no 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.