[std] Rephrase notes containing 'must'

[jensmaurer]

Fixes ISO/CS 037 (C++23 DIS).

Fixes cplusplus/nbballot#579

[jensmaurer]

@tkoeppe , this is an initial sample of what might be done. Please have a look.

[tkoeppe]

This is great, thank you! By and large this even looks like an improvement, don't you think?

[jensmaurer]

Major update; now all mentions of "must" (except in a literal string in an example) are gone.

@tkoeppe, are we good now?

[tkoeppe]

Great! But please note that we can't say "required" as per drafting directives, since that's just a synonym for "shall" :-(

[jensmaurer]

Yes, but "required" is mostly in Annex C, not directly in notes, so maybe they don't notice.

(Frankly, rephrasing Annex C to avoid "required" is a major undertaking, given that it wants to highlight differences between rules.)

[tkoeppe]

I'll rebase and apply this PR after the IS has shipped. I'm pushing additonal changes I made to the IS back into this PR, too, so we don't lose them.