Fix macros inside backticks

[ntrel]

Fixes Issue 23849 - Ddoc $(LPAREN) and $(RPAREN) not formatted properly

Introduced by #6391 in 2018.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23849	enhancement	Ddoc $(LPAREN) and $(RPAREN) not formatted properly

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8739"

[ntrel]

Some of these changes are wrong with the current ddoc - I thought macros weren't expanded inside backticks, and they aren't for the filed issue. It seems in some cases they are expanded. The spec doesn't seem clear on the behaviour:
https://dlang.org/spec/ddoc.html#inline_code

Text inside these sections will be escaped according to the rules described above

[ntrel]

@adamdruppe:

Backticks can not contain embedded HTML (and are not supposed to contain macros, the way I intended it, but the implementation didn't keep to that intention)

#5183 (comment)

Is it intended for macro expansion to always happen inside backticks?

[ntrel]

Ah I see what the problem is, ddoc actually always expands macros but ddox doesn't for the issue code.

[ntrel]

Filed dlang/ddox#241.