Flat map assorted editorial improvements

[tkoeppe]

See #5626 for remaining, unaddressed suggestions.

@jwakely, @CaseyCarter ^^^

[tkoeppe]

We should consider styling the "exposition-only" members of flat*_map as \exposid.

[JohelEGP]

We should consider styling the "exposition-only" members of flat*_map as \exposid.

Please, see #5696 (comment).

[JohelEGP]

Might #5626 (comment) and #5626 (comment) need a LWG issue? Those deduction guides are useless. So changing that might not classify as editorial.

[tkoeppe]

Might #5626 (comment) and #5626 (comment) need a LWG issue? Those deduction guides are useless. So changing that might not classify as editorial.

I certainly want to hear from the LWG chair about this before moving further.

[JohelEGP]

Here's some refactoring for simpler wording.
-- #5626 (review)

https://github.com/cplusplus/draft/pull/5626/files#r922759807: What is range? e.first might be ill-formed.

#5626 (comment): Inconsistent handling of unmet complexity guarantees.

#5626 (comment) should be harmonized with #5626 (comment).

#5626 (comment): C is not an iterator.

[tkoeppe]

@jwakely Could you please take a look at those "useless deduction guides" above?

[JohelEGP]

I opened #5800 just for that.