gh-107862: Note why the folding strategy avoids encoded word look-alikes

The round trip property cannot cover text that is itself a well-formed
encoded word, because such text is written out unencoded and decodes to
something else on reparse. That is now tracked separately as gh-160148,
so point at it from the strategy rather than carrying an expected
failure here.

Author: pganssle

Lib/test/test_email/test_policy.py

@@ -308,7 +308,8 @@ def test_wide_char_too_long_for_line_does_not_cause_inf_loop(self):
     # Pieces chosen to exercise folding: words, runs of whitespace, RFC 5322
     # specials, characters needing 2, 3 and 4 byte UTF-8 encodings, and the
     # delimiters of RFC 2047 encoded words.  The pieces cannot combine into a
-    # well-formed encoded word (there is no lone '?').
+    # well-formed encoded word (there is no lone '?'), since such text is
+    # not encoded and so does not round trip; that is gh-160148.
     _fold_pieces = hypothesis.strategies.sampled_from([
         'a', 'word', 'longwordlongwordlongword', ' ', '  ', '\t', ',', ';',
         ':', '(', ')', '<', '>', '"', '@', '\\', '.', '-', '_', '\xa0',