Document let-punning in the manual

[stedolan]

Missing docs from #10013

[Octachron]

You should also update the grammar at the start of the section on binding operators.

[gasche]

Thanks! This looks mostly good, see proposals for improvement.

[stedolan]

@Octachron Good point, done.

The manual currently incorrectly documents the RHS of a let operator as a let-binding. There are three cases of let-binding:: value bindings, function bindings and polymorphic bindings.

• Value bindings are the normal case let* x = foo in ...
• Function bindings let* f x = ... in ... work but are not very useful. (In monadic code, they can only yield types like (a -> b) Monad.t, and can never be polymorphic)
• Polymorphic bindings let* f : 'a . 'a -> 'a = ... in ... are a syntax error

I've defined letop-binding to include value bindings and punning, but left out the other two.

[Octachron]

It seems fine to let the let* f x = ... case out of the simplified grammar described in the manual.