Update fix_ampersands documentation for behavior with existing entities

[Jerry]

fix_ampersands doesn’t replace all ampersands with & entities; it attempts, usually successfully, to exclude named entities and numeric character references while converting all ampersands that do need replacing. This (and its behavior in some edge cases, noted at ticket:16066) should be documented.

Adds two lines under fix_ampersands in docs/_build/html/ref/templates/builtins.html:

Named entities and numeric character references are not replaced. If ``value``
is ``Pepé Le Pew``, the output will remain ``Pepé Le Pew``.

In some edge cases--such as abbreviations followed by semicolons--this filter
will not replace ampersands that need replacing. If ``value`` is ``Darrin, run
that past R&D; Betty…``, the output will remain unchanged because ``&D;``
resembles a named entity.

[jerry]

updates fix_ampersands documentation to note its behavior when entities or entity-like characters already exist in the text.

[Julien Phalip]

Thanks, this is good to go. I've just made some minor edits.

[Julien Phalip]

Fixed typo

[Jannis Leidel]

In [16431]:

(The changeset message doesn't reference this ticket)