all: introduce isize and usize (#11437)

doc/docs.md

@@ -419,6 +419,8 @@ rune // represents a Unicode code point
 
 f32 f64
 
+isize, usize // platform-dependent, the size is how many bytes it takes to reference any location in memory
+
 voidptr, size_t // these are mostly used for C interoperability
 
 any // similar to C's void* and Go's interface{}