quote YAML strings starting with % or @ characters

Pygment uses the YAML 1.1 standard to parse YAML code being highlighted.
A string value cannot start with the percent sign or the @ character
in YAML 1.1 without being quoted. To highlight the YAML code blocks
properly, strings starting with the percent sign are now enclosed in
double quotes.
remove "..." from XML element tags

The ellipses were used as placeholders in XML element tags. Thus, the
XML wasn't well-formed and therefore would have never been rendered
properly.
minor #4030 enclose YAML strings containing % with quotes (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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enclose YAML strings containing % with quotes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #1886, #4029

Thanks @iamdto, did you spot some other occurences in the docs?

Commits
-------

c1c4bc8 remove "..." from XML element tags
dbc02a2 quote YAML strings starting with % or @ characters