Fix Issue 20508 - std.math.pow(-infinity, y) does not return NaN for imaginary or complex results

[berni44]

This is a copy of #7363. I'm the original author, but as I deleted my old account, I can't do any changes there.

I rebased everything and added the requested changes to the documentation unittest.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20508	normal	std.math.pow(-infinity, y) does not return NaN for imaginary or complex results

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#7747"

[RazvanN7]

LGTM. cc @wilzbach