Consider documenting the "location" of a node

[bakkot]

Followup to #69:

Various tools (e.g shift-parser-js) provide a way of querying the location corresponding to a node in some source text. We might consider saying what that means, at least for edge cases, such as:

• do FormalParameters include parentheses?
• do Programs include leading and/or trailing whitespace? what about when there are no non-whitespace tokens?
• where do grouping parentheses go? e.g. in (a + b);, are they in the BinaryExpression or the ExpressionStatement?