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.
(cherry picked from commit eb97b9211e7c99841d6cae8c63893b3525d5a401)

Co-authored-by: tmblweed <tmblweed@users.noreply.github.com>

2 files changed, 6 insertions, 3 deletions

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 @@ 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():
diff --git a/Lib/textwrap.py b/Lib/textwrap.py
index 8103f34..30e693c 100644
--- a/Lib/textwrap.py
+++ b/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.