[Issue 18361] Ddoc: support ability to opt out of automatic keyword highlighting in text

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18361

--- Comment #1 from hsteoh at quickfur.ath.cx ---
Currently, the automatically highlighted text is put into the DDOC_PSYMBOL,
DDOC_KEYWORD, and DDOC_PARAM macros.  If the automatic highlights were to be
replaced with DDOC_AUTO_PSYMBOL, DDOC_AUTO_KEYWORD, DDOC_AUTO_PARAM instead,
with the default definitions of these macros pointing to the foregoing three
respective macros, then it would provide the user a way to override this
behaviour, e.g., by redefining these macros to just return the raw text
instead.

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