Update trait summaries for std::fmt

[brennie]

This patch is part of #29355.

r? @steveklabnik

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[brennie]

I'm not sure if the sub-task I was intending to tackle (all of the traits should say "C formatting", where C is the char, rather than their current boilerplate. Except Write. And Display.) was only about the summaries. If further revision of the trait docs are required I can attempt to give it a go.

[steveklabnik]

This looks great, thanks so much!

@bors: r+ rollup

[bors]

📌 Commit 276bebe has been approved by steveklabnik