Docs unclear whether `parser!` should be used on nightly rust

[spencerwhite]

Docs unclear whether `parser!` should be used on nightly rust

[Marwes]

impl Parser can be used for most parsers regardless of whether you use stable or nightly now (the comment is outdated). However you do need to use parser! or opaque! to construct parsers which contain itself to prevent the impl Parser type from containing itself (which is impossible).

combine/benches/json.rs

Lines 165 to 166 in 24be5ab

	// We need to use `parser!` to break the recursive use of `value` to prevent the returned parser
	// from containing itself

[spencerwhite]

Alright, thanks