std.regex: document that ctRegex always uses the backtracking engine

[kubo39]

ctRegex pins CtfeFactory!(BacktrackingMatcher, ...) via withFactory, bypassing the runtime engine auto-selection that picks the Thompson matcher for patterns without backreferences. There is no compile-time code generator for the Thompson engine, so ctRegex matching carries no linear-time guarantee. Document this trade-off so users handling untrusted input can make an informed choice.

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[DmitryOlshansky]

Yes, it's fair to warn about the nature of the engine being used.