Fix Issue 18319 - std.exception: enforce example does not compile

[wilzbach]

I used the opportunity to improve the docs and unify all three overloads under one documentation block.

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

Auto-close	Bugzilla	Description
✓	18319	std.exception: enforce example does not compile

[wilzbach]

Note that enforcEx has a similar problem, but that's just historical baggage that should be deprecated or at least undocumented (see #5234).

[quickfur]

LGTM