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[3.14] gh-135676: Lexical analysis: Reword String literals and related sections (GH-135942) (#137048)

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Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Blaise Pabon <blaise@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
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Doc/reference/introduction.rst
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@ -145,15 +145,23 @@ The definition to the right of the colon uses the following syntax elements:
* ``e?``: A question mark has exactly the same meaning as square brackets:
the preceding item is optional.
* ``(e)``: Parentheses are used for grouping.
The following notation is only used in
:ref:`lexical definitions <notation-lexical-vs-syntactic>`.
* ``"a"..."z"``: Two literal characters separated by three dots mean a choice
of any single character in the given (inclusive) range of ASCII characters.
This notation is only used in
:ref:`lexical definitions <notation-lexical-vs-syntactic>`.
* ``<...>``: A phrase between angular brackets gives an informal description
of the matched symbol (for example, ``<any ASCII character except "\">``),
or an abbreviation that is defined in nearby text (for example, ``<Lu>``).
This notation is only used in
:ref:`lexical definitions <notation-lexical-vs-syntactic>`.
.. _lexical-lookaheads:
Some definitions also use *lookaheads*, which indicate that an element
must (or must not) match at a given position, but without consuming any input:
* ``&e``: a positive lookahead (that is, ``e`` is required to match)
* ``!e``: a negative lookahead (that is, ``e`` is required *not* to match)
The unary operators (``*``, ``+``, ``?``) bind as tightly as possible;
the vertical bar (``|``) binds most loosely.