break Validity Invariant and Safety Invariant apart

Author: Lokathor

reference/src/glossary.md

@@ -14,7 +14,7 @@ If data immediately pointed to by a `*const T` or `&*const T` is mutated, that's
 *Interior mutability* refers to the ability to perform interior mutation without causing UB.
 All interior mutation in Rust has to happen inside an [`UnsafeCell`](https://doc.rust-lang.org/core/cell/struct.UnsafeCell.html), so all data structures that have interior mutability must (directly or indirectly) use `UnsafeCell` for this purpose.
 
-#### Validity and safety invariant
+#### Validity Invariant
 
 The *validity invariant* is an invariant that all data must uphold any time it is accessed or copied in a typed manner.
 This invariant is known to the compiler and exploited by optimizations such as improved enum layout or eliding in-bounds checks.
@@ -31,6 +31,8 @@ fn main() { unsafe {
 } }
 ```
 
+#### Safety Invariant
+
 The *safety* invariant is an invariant that safe code may assume all data to uphold.
 This invariant is used to justify which operations safe code can perform.
 The safety invariant can be temporarily violated by unsafe code, but must always be upheld when interfacing with unknown safe code.