Specify that the types at least have one niche

Author: gnzlbg

reference/src/layout/enums.md

@@ -323,7 +323,8 @@ as a niche.
 
 The niche values of a type are parts of its validity invariant which, as of this
 writing, is the current active discussion topic in the unsafe code guidelines
-process. [rust-lang/rust#60300] specifies that the following types have a niche:
+process. [rust-lang/rust#60300] specifies that the following types have at least
+one niche (the all-zeros bit-pattern):
 
 * `&T`
 * `&mut T`