doc: a few minor style fixes

For crates `alloc`–`collections`. This is mostly just updating a few function/method descriptions to use the indicative style. 

cc #4361; I’ve sort of assumed that the third-person indicative style has been decided on, but I could update this to use the imperative style if that’s preferred, or even update this to remove all function-style-related changes. (I think that standardising on one thing, even if it’s not the ‘best’ option, is still better than having no standard at all.) The indicative style seems to be more common in the Rust standard library at the moment, especially in the newer modules (e.g. `collections::vec`), more popular in the discussion about it, and also more popular amongst other languages (see https://github.com/rust-lang/rust/issues/4361#issuecomment-33470215).
I would just say "mutable slice"