Improve *Foreach over Tuples* spec

[ntrel]

• Compile-time Tuple -> Sequence (see Improve Compile-time sequences docs #2204)
• ExpressionTuple -> ValueSeq (this matches https://dlang.org/spec/template.html#variadic-templates point 2)
• Improve wording of Foreach over Tuples & example
• Add link to static foreach

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[wilzbach]

LGTM

[wilzbach]

It can also be long: https://run.dlang.io/is/kxSAFP as the index is size_t

[ntrel]

@wilzbach I've added long and ulong here, and updated runtime foreach index types, which listed just size_t instead of these. BTW for runtime foreach only, dmd accepts short and byte - I've not documented this though.