gh-80143: Clarify use of backslash in string literals

[nw0]

This reworks the sentence described in the issue. I have omitted the words "special meaning" in favour of "escape sequences" and examples, linking to the table of escape sequences -- I hope this is clearer or at least "plain English".

[kumaraditya303]

Docs changes don't require a news entry. You can remove it, I added skip news label.

[carljm]

I think the cross-link to the escape sequences reference is an improvement. I'm not sure that replacing "characters that otherwise have a special meaning" with "characters that may not otherwise appear in the literal" is an improvement. The latter may be misleading, since it could be misunderstood to mean that all characters with escape sequences are otherwise always invalid in string literals. This is not true: e.g. ' can be escaped, but it is also valid unescaped in a string literal enclosed in " or triple-quoted. Similar for newline, which is valid unescaped in a triple-quoted literal.

I think "characters that might not otherwise be allowed in the literal" could work, though tbh I think the existing "characters that otherwise have a special meaning" wording is still just as good or slightly better. It could be improved by clarifying to "characters that otherwise may have a special meaning."

[carljm]

I think #92292 is probably a better option here; it also adds the clarification that some escape sequences (like \n) involve characters that otherwise wouldn't be in any way special!