Document `private` keyword in `module type`

[quinn-dougherty]

Example

[fhammerschmidt]

There is also this idiom:

type element = private {}

Which can be used instead of an abstract type in order for the compiler to not unnecessarily add Primitive_option.some calls for optional props.

Explanation: https://github.com/rescript-lang/rescript/pull/7450/files#r2081532393