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.
diff --git a/Doc/library/textwrap.rst b/Doc/library/textwrap.rst
index d254466..0f11ef4 100644
--- a/Doc/library/textwrap.rst
+++ b/Doc/library/textwrap.rst
@@ -77,6 +77,9 @@
    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():