Improve documentation of slicing methods.

[cassiersg]

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[jturner314]

Thanks for suggesting improvements to the docs.

It's somewhat awkward to reference a few other slicing methods without listing all of the potentially relevant ones. However, I don't think we should list all of them, since there are a lot of slicing methods, and they're all mentioned in the Slicing docs already.

What I'd suggest is to address the question in #988 more directly, i.e. add something like the following to the docs for .slice():

Note that the lifetime of the resulting slice corresponds to the &self borrow. If you want a slice with the same lifetime as an existing view, clone/copy the view if necessary and then use .slice_move() or .slice_collapse().

and add the following to the docs for .slice_mut():

Note that the lifetime of the resulting slice corresponds to the &mut self borrow. If you want a slice with the same lifetime as an existing view, use .slice_move(), .multi_slice_move(), or .slice_collapse() on the view.

We could also add analogous comments to the docs of slice_axis/_mut (referencing slice_axis_inplace) and slice_each_axis/_mut (referencing slice_each_axis_inplace).

One other note: the markup of the links in this PR isn't quite right. A link should be formatted like this:

some text [the link text](url) more text

or like this:

some text [the link text] more text

[the link text]: url

Note also that as of Rust 1.48.0, it's possible to use the Rust path of an in-scope item as a "URL", and as of Rust 1.51.0, this works for associated items too, such as writing [.slice()](ArrayBase::slice).