Improve documentation of `TagEncoding`
This PR is follow-up from the [discussion here](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/.E2.9C.94.20VariantId.3DDiscriminant.20when.20tag.20is.20niche.20encoded.3F/with/524384295).
It aims at making the `TagEncoding` documentation less ambiguous and more detailed with references to relevant implementation sides. It especially clears up the ambiguous use of discriminant/variant index, which sparked the discussion referenced above.
PS: While working with layout data, I somehow ended up looking at the docs for `FakeBorrowKind` and noticed that the one example was not in a doc comment. I hope that this is minor enough of a fix for it to be okay in this otherwise unrelated PR.
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