merge and reword the whole junk mechanism

Author: Lenormju

Doc/library/difflib.rst

@@ -40,29 +40,18 @@ diffs. For comparing directories and files, see also, the :mod:`filecmp` module.
    complicated way on how many elements the sequences have in common; best case
    time is linear.
 
-   **User-defined junk predicate**: :class:`SequenceMatcher` accepts an *isjunk*
-   parameter to let users determine which lines should be ignored while computing
-   the diff.
-
-   .. note::
-
-      This predicate only ever inspects the second sequence for junk. That has
-      an unexpected consequence: swapping the inputs can produce a different
-      diff than simply reversing the output, sometimes very large.
-
-   **Automatic junk heuristic:** :class:`SequenceMatcher` supports a heuristic that
-   automatically treats certain sequence items as junk. The heuristic counts how many
-   times each individual item appears in the sequence. If an item's duplicates (after
-   the first one) account for more than 1% of the sequence and the sequence is at least
-   200 items long, this item is marked as "popular" and is treated as junk for
-   the purpose of sequence matching. This heuristic can be turned off by setting
-   the ``autojunk`` argument to ``False`` when creating the :class:`SequenceMatcher`.
-
-   .. note::
-
-      This heuristic only ever inspects the second sequence for junk. That has
-      an unexpected consequence: swapping the inputs can produce a different
-      diff than simply reversing the output, sometimes very large.
+   **Junk**: :class:`SequenceMatcher` accepts an ``isjunk`` predicate and an
+   ``autojunk`` flag. Items that are considered as junk will not be considered
+   to find similar content blocks. This can produce better results for humans
+   (typically breaking on whitespace) and faster (because it reduces the number
+   of possible combinations). But it can also cause pathological cases where
+   too many items considered junk cause an unexpectedly large (but correct)
+   diff result.
+   You should consider tuning them or turning them off depending on your data.
+   Moreover, only the second sequence is inspected for junk. This causes the diff
+   output to not be symmetrical.
+   When ``autojunk=True``, it will consider as junk the items that account for more
+   than 1% of the sequence, if it is at least 200 items long.
 
    .. versionchanged:: 3.2
       Added the *autojunk* parameter.