Rollup merge of rust-lang#32906 - jocki84:jocki84-book-size, r=steveklabnik

Reword explanation of 'size' types.

Do not reference machine 'pointers' in explanation of 'size' types.

I think the number of elements that can be directly addressed is a fundamental feature of a machine architecture in its own right. The fact that it coincides with the ‘size’ of a pointer should be viewed as an ‘implementation detail’ ;)

Author: steveklabnik

src/doc/book/primitive-types.md

@@ -97,9 +97,10 @@ and `i64` is a signed, 64-bit integer.
 
 ## Variable-size types
 
-Rust also provides types whose size depends on the size of a pointer of the
-underlying machine. These types have ‘size’ as the category, and come in signed
-and unsigned varieties. This makes for two types: `isize` and `usize`.
+Rust also provides types whose particular size depends on the underlying machine
+architecture. Their range is sufficient to express the size of any collection, so
+these types have ‘size’ as the category. They come in signed and unsigned varieties
+which account for two types: `isize` and `usize`.
 
 ## Floating-point types