Auto merge of #41084 - QuietMisdreavus:rustdoc-format-redux, r=frewsxcxv,GuillaumeGomez

rustdoc: update formatting of fn signatures and where clauses to match style rfcs

Recent updates to style RFCs ([where clauses](https://github.com/rust-lang-nursery/fmt-rfcs/issues/38), [function definitions](https://github.com/rust-lang-nursery/fmt-rfcs/issues/39)) changed the "canonical" style for these items, so this is a rustdoc update to make it emit that style where necessary. This is mainly a conversion from visual indent to block indent, which helps out in situations where there was excessive indent causing lines to wrap regardless.

Samples:

![std::iter::IntoIterator](https://cloud.githubusercontent.com/assets/5217170/24712947/e586604c-19e9-11e7-87ae-4fe64d689dc3.png)

![excerpt from std::iter::Iterator](https://cloud.githubusercontent.com/assets/5217170/24713209/91e65112-19ea-11e7-9ff8-d4cf6b31aae1.png)

![std::iter::FromIterator](https://cloud.githubusercontent.com/assets/5217170/24713138/59f36114-19ea-11e7-9dbb-5f5ba7126e2e.png)

![std::cmp::min](https://cloud.githubusercontent.com/assets/5217170/24713038/1bab88b4-19ea-11e7-935d-defed5648de4.png)

![some trait impls on std::collections::HashMap](https://cloud.githubusercontent.com/assets/5217170/24713251/b7ef69e8-19ea-11e7-94a7-e01fbf89fa31.png)

![`fn extract_code_blocks`, an example given in #40687](https://cloud.githubusercontent.com/assets/5217170/24713159/672717cc-19ea-11e7-9acb-6ac278b90339.png)

![excerpt from itertools::Itertools](https://cloud.githubusercontent.com/assets/5217170/24713323/f06716ea-19ea-11e7-94cc-6ef68d9980ec.png)

fixes #41025 and #40687

r? @rust-lang/docs

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