Fix and clarify cases in ignore patterns (#3583)

Co-authored-by: Eric Larson <larson.eric.d@gmail.com>

Author: DanielYang59

README.rst

@@ -119,7 +119,7 @@ You can select the optional dictionaries with the ``--builtin`` option.
 Ignoring words
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-When ignoring false positives, note that spelling errors are *case-insensitive* but words to ignore are *case-sensitive*. For example, the dictionary entry ``wrod`` will also match the typo ``Wrod``, but to ignore it you must pass ``Wrod``.
+When ignoring false positives, note that spelling errors are *case-insensitive* but words to ignore are *case-sensitive*. For example, the dictionary entry ``wrod`` will also match the typo ``Wrod``, but to ignore it you must pass ``wrod`` (to match the case of the dictionary entry).
 
 The words to ignore can be passed in two ways: