Document that slice refers to any pointer type to a sequence

[LeonMatthes]

I was recently confused about the way slices are represented in memory. The necessary information was not available in the std-docs directly, but was a mix of different material from the reference and book.

This PR should clear up the definition of slices a bit more in the documentation. Especially the fact that the term slice refers to the pointer/reference type, e.g. &[T], and not [T].
It also documents that slice pointers are twice the size of pointers to Sized types, as this concept may be unfamiliar to users coming from other languages that do not have the concept of "fat pointers" (especially C/C++).

I've documented why this was important to me and my findings in this blog post.

r? @lcnr

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @lcnr (or someone else) soon.

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[lcnr]

I am not comfortable with approving this myself.

I think someone from the docs team should look over this

[lcnr]

r? @steveklabnik to review/reassign

[Dylan-DPC-zz]

@MrModder any updates on this?

[LeonMatthes]

@Dylan-DPC . The changes to the first paragraph are now reverted.

Hopefully the PR can be merged now.

[steveklabnik]

@bors: r+ rollup

thank you! Sorry this took a while :/

[bors]

📌 Commit cf76256 has been approved by steveklabnik

[bors]

⌛ Testing commit cf76256 with merge d02a209...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: steveklabnik
Pushing d02a209 to master...