Update NOTE about `ts_node_is_missing()` support

[DavisVaughan]

With new insight from tree-sitter/tree-sitter#3332

tree-sitter-r/grammar.js

Lines 524 to 530 in 6c52336

	// NOTE: Due to the linked tree-sitter bug, if `_identifier` and `_quoted_identifier`
	// are their own hidden rules, then we can't detect implied `identifier`s as missing
	// with `ts_node_is_missing()`, essentially making that function useless since
	// tree-sitter seems to fill in named missing nodes with `identifier` most of the
	// time. The workaround used here inlines the regexes, and wraps the `choice()` call
	// in a single terminal `token()` so `identifier` can still be used as the `word` rule.
	// https://github.com/tree-sitter/tree-sitter/issues/3332