clarify scoping rules

[ckp95]

The spec doesn't lay out the precise rules for variable scoping. I didn't find any surprises but I still had to discover them by experimentation. This is what I gathered:

• lexical block scoping for functions, methods, if/case/for/while etc
• a new variable defined in a if/case/for/while does not outlive the block even if it has the same name and type in all branches
• you can't use let to re-define an existing variable, even if it was made in an outer scope (no shadowing)
• if a linear variable is borrowed in a borrow block, it vanishes for the duration of the borrow block then reappears afterwards

Is this a fair description?

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oh also the let-destructure section doesn't talk about the optional as fragment.