Auto merge of #41286 - lukaramu:std-collections-docs, r=frewsxcv

Various improvements in std::collections docs

The meat of this PR are:
* changes to (almost all?) iterator struct docs in std::collections such that they use the standard iterator boilerplate and state where they are created
* a bunch of added links (at least as much as possible given std::collections mostly being a facade and whatnot :sweat_smile:)
* an example for `Bound`
* changed phrasing for some summary sentences to be less redundant as well as more consistant with others in the module

There also are various other fixes, e.g. removing parens from method names in the module docs, changing some imperatives to 3rd person, etc.

r? @steveklabnik

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