`bulk` doesn't explicitly grant permissions for customisations to execute `f` concurrently

[lewissbaker]

The wording for [exec.bulk] in P2300R10 only talks about the default implementation and provides a description of the default sequential implementation.

It does not explicitly grant or deny customizations the ability to invoke the function concurrently.

I think we need to say something about the constraints we want to place on customizations:

• specify when they are allowed to execute the function concurrently (see also bulk doesn't give the caller a way to constrain how function can be executed #275)
• specify whether the function-object is allowed to be copied (see also [execution.senders.adaptors.bulk] Should clarify that bulk customisations are allowed to decay-copy 'args' if required #103)
• specify which execution context it is allowed to run on
• specify how exceptions thrown by f should be handled (see also bulk does not specify what customizations need to do with exceptions thrown from invocations of f #276)

[inbal2l]

Issue is addressed in: "P3481: Summarizing std::execution::bulk() issues" (Lewis Baker, Lucian Radu Teodorescu, Ruslan Arutyunyan)