Improve CTFE section

[Bolpat]

The most notable change is “Expressions may not reference any global or local static variables” to “Expressions may not reference any mutable static variables”. That is because immutable static variables are fine (in theory and practice) and it doesn’t matter where the static variable is declared.

The other notable change is from “The function must have a SpecifiedFunctionBody.” to “The function must have a FunctionBody.” That is because it is in fact a FunctionBody that’s required (in theory and practice).

Everything else is just wording and typography.

[dlang-bot]

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