Being clearer about use of RFC 2119

Start with purpose_and_scope.html and RFC 2119, then review the standard's uses of normative terms such as “should,” “must,” “may,” and “optional.” Identify where the wording or presentation causes ambiguity, and define completion as an agreed RFC 2119 explanation and consistent terminology or formatting throughout the standard.

We use RFC 2119 for words like "should" and "must" in the standard. But many people are confused by this and think that "should" restrictions are actually "must" restrictions. This is most common for dtype restrictions, but it appears in other places in the standard as well.

We need to be clearer about this somehow. Some suggestions

• Capitalize (as is the common convention) or bold these words (https://github.com/data-apis/array-api/issues/397)

• Add a section somewhere making it clear that we are following the RFC. There is a link to the RFC at the bottom of https://data-apis.org/array-api/latest/purpose_and_scope.html but that's it. The RFC itself says:

  Authors who follow these guidelines
  should incorporate this phrase near the beginning of their document:

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
  NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
  "OPTIONAL" in this document are to be interpreted as described in
  RFC 2119.

• The RFC also lists "required", "shall", "recommended", "may", and "optional". We should review our use of those terms to make sure they match the RFC.

• We might be using "should" in places where we should be using "may" or "optional". From the RFC: SHOULD "means that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course", but MAY "means that an item is truly optional". Things like data type restrictions are probably correct in using SHOULD, but things like indexing restrictions, like out-of-bounds slices" perhaps ought to use MAY, since the only reason they are limited is so the the standard can support libraries that can't implement them.

• We should consider if there are other ways things could be made clearer, for instance, explicitly stating "other behavior is unspecified".