Add note clarifying 16-bit support.

Author: ehuss

src/types/numeric.md

@@ -40,3 +40,8 @@ differences between pointers into an object or array and can address every byte
 within an object along with one byte past the end.
 
 `usize` and `isize` are at least 16-bits wide.
+
+> **Note**: Many pieces of Rust code may assume that pointers, `usize`, and
+> `isize` are at least 32-bit and at most 64-bit. As a consequence, 16-bit
+> pointer support is limited and may require explicit care and acknowledgment
+> from a library to support.