Remove use of undefined ddoc macros from Phobos

[wilzbach]

This is required for dlang/dmd#7834
The uses of _ has been wrong in both cases.

CC @quickfur

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[quickfur]

Whoa, what's with the ddoc test failure?

[quickfur]

I don't think you want to use π here; it will break LaTeX generation, won't it?

[wilzbach]

Whoa, what's with the ddoc test failure?

Argh damn. We still use the old Ddoc behavior here. I expanded the PI macro - an alternative would have been to rename it.

[quickfur]

But what about the other lines that reference $(PI)? How come they compile, but this line doesn't?

[wilzbach]

it will break LaTeX generation, won't it?

We only generate LaTeX from the spec, not the documentation.
I just did inserted, what Ddoc would have done manually:

phobos/std/math.d

Line 106 in b3ebd07

* PI = π

But as mentioned I can also rename the macro if you prefer this.

[quickfur]

Ohhh I know why... because of the stupid auto-highlight, since PI is being defined, it needs to be escaped otherwise it will mangle the macro. :-(

[wilzbach]

But what about the other lines that reference $(PI)? How come they compile, but this line doesn't?

Auto-escaping - the thing you want to change in your PR ;-)

[quickfur]

Haha, yeah. This is why we need to kill that misfeature with fire. :-( I guess we should just go with π for now.

[wilzbach]

Yes I think so too.
Once you PR is in, we can easily go back to PI (though honestly it doesn't matter).