Added minor clarification to specification of realloc.
Added minor clarification to specification of realloc.

The `layout` for the returned allocation of a `realloc` is
only implicitly specified.  This change makes it explicit.

graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust