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)

1 files changed, 8 insertions, 0 deletions

diff --git a/src/libstd/primitive_docs.rs b/src/libstd/primitive_docs.rs
index 76ef36cc9a7..1edb35d8fe7 100644
--- a/src/libstd/primitive_docs.rs
+++ b/src/libstd/primitive_docs.rs
@@ -710,6 +710,10 @@ mod prim_u128 { }
 //
 /// The pointer-sized signed integer type.
 ///
+/// The size of this primitive is how many bytes it takes to reference any
+/// location in memory. For example, on a 32 bit target, this is 4 bytes
+/// and on a 64 bit target, this is 8 bytes.
+///
 /// *[See also the `std::isize` module](isize/index.html).*
 ///
 /// However, please note that examples are shared between primitive integer
@@ -722,6 +726,10 @@ mod prim_isize { }
 //
 /// The pointer-sized unsigned integer type.
 ///
+/// The size of this primitive is how many bytes it takes to reference any
+/// location in memory. For example, on a 32 bit target, this is 4 bytes
+/// and on a 64 bit target, this is 8 bytes.
+///
 /// *[See also the `std::usize` module](usize/index.html).*
 ///
 /// However, please note that examples are shared between primitive integer