Clarify proc-macro token tree

[ehuss]

This attempts to clarify some details about proc-macro tokens, in particular the differences in handling _ from macro_rules. This includes several changes:

• Removed the wordy description of a macro_rules TokenTree. This is already defined in the grammar in more detail. This does remove the sentence discussing metavariable substitutions. I would prefer to defer this to a more detailed description in the macro_rules chapter on how metavariable substitutions work.
• Removed the English description of a proc-macro token tree, and instead write it as a grammar production.

Closes https://github.com/rust-lang/reference/issues/2020 Closes https://github.com/rust-lang/reference/issues/1236

[petrochenkov]

r? @petrochenkov

[petrochenkov]

@rustbot author (I don't have rights to change labels directly here.)

[rustbot]

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