Reference syntax for Closure type is incomplete

[jsgf]

The Reference says the syntax for a closure type is:

closure_type := [ 'unsafe' ] [ '<' lifetime-list '>' ] '|' arg-list '|'
            [ ':' bound-list ] [ '->' type ]
arg-list := ident ':' type | ident ':' type ',' arg-list
...

arg-list seems incomplete as it doesn't cover |: ...| |&: ...| |&mut: ...|

The Reference only incidentally mentions |&: ...| and doesn't touch any of the other forms, and doesn't describe their semantics.

[frewsxcv]

Relevant issues: #19841 #20667

[steveklabnik]

Closing as a dup of #19841