Rollup merge of #44648 - Havvy:doc-size_of, r=dtolnay

Expand size_of docs

This PR does 3 things.

1. Adds a description of what pointer size means to the primitive pages for usize and isize.
2. Says the general size of things is not stable from compiler to compiler.
3. Adds a table of sizes of things that we do guarantee. As this is the first table in the libstd docs, I've included a picture of how that looks.

![](https://i.imgur.com/YZ6IChH.png?1)

diff --cc src/libcore/mem.rs
Simple merge