- Clarify the crlf does not count towards line length in fold

(LP: #1823008)

ChangeLog

@@ -7,6 +7,9 @@ UNRELEASED Version 0.9.2
     - Don't log message headers and body unless explicitely requested. This
       should also reduce memory usage on large messages. (Jonathan
       Bastien-Filiatrault).
+    - Clarify the crlf does not count towards line length in fold
+      (LP: #1823008)
+
 2018-12-09 Version 0.9.1
     - Fixed ARC verification to fail if h= tag is present in Arc-Seal and
       added tests

dkim/__init__.py

@@ -368,7 +368,8 @@ def text(s):
 
 
 def fold(header, namelen=0):
-    """Fold a header line into multiple crlf-separated lines at column 72.
+    """Fold a header line into multiple crlf-separated lines of text at column
+     72.  The crlf does not count for line length.
 
     >>> text(fold(b'foo'))
     'foo'
@@ -382,6 +383,8 @@ def fold(header, namelen=0):
     'x'
     >>> text(fold(b'xyz'*24))
     'xyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyz'
+    >>> len(fold('abxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxyzxy\\r\\n'))
+    78
     """
     # 72 is the max line length we actually want, but the header field name
     # has to fit in the first line too (See Debian Bug #863690).