Start to opt-out of Ddoc's automatic keyword highlighting

[wilzbach]

Let's whether DAutoTest likes this. In theory

• no changes should show up in the diff
• existing suppression of the auto-highlighting can be gradually removed (e.g. with the SUPPRESS helper or grep)
• ddoc should no longer do automatic highlighting (not sure whether we can verify this here, but @JackStouffer just run into this issue again - Simplified std.datetime.package overview phobos#6356)

See also:

• https://dlang.org/changelog/2.079.0.html#fix18361
• Fix issue 18361: [ddoc] redirect automatically highlighted text in re… dmd#7834

CC @quickfur

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[andralex]

why?

[wilzbach]

Because DDoc doesn't support comments and DDOC_AUTO_PSYMBOL_SUPPRESS is within reach. After all, there are still tons of underscores used to suppress auto highlighting like _range references in Phobos.