More self-explanatory pragmas for inlining phase control

Eventually implement https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/self-explinatory-inline-pragmas.

And while at it: Extend the docs where they say “You shouldn't ever need to do this, unless you're very cautious about code size.”

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Trac field	Value
Version	7.10.2
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
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