Re: Missing comments/docs about custom scan path

Another thing I would like to propose is minor adjustments to the docs
related to parallel query:

    A custom scan provider will typically add paths for a base relation by
    setting the following hook, which is called after the core code has
    generated all the access paths it can for the relation (except for
    Gather paths, which are made after this call so that they can use
    partial paths added by the hook):

For clarity, I think "except for Gather paths" should be "except for
Gather and Gather Merge paths".

    Although this hook function can be used to examine, modify, or remove
    paths generated by the core system, a custom scan provider will
    typically confine itself to generating CustomPath objects and adding
    them to rel using add_path.

For clarity, I think "adding them to rel using add_path" should be eg,
"adding them to rel using add_path, or using add_partial_path if they
are partial paths".