more explicitly explain the UB around immutable extern statics

Author: RalfJung

src/behavior-considered-undefined.md

@@ -50,7 +50,7 @@ Please read the [Rustonomicon] before writing unsafe code.
   types are passed in a (nested) field of a compound type, but not behind
   pointer indirections.
 * Mutating immutable bytes. All bytes inside a [`const`] item are immutable.
-  The bytes owned by an immutable binding are immutable, unless those bytes are part of an [`UnsafeCell<U>`].
+  The bytes owned by an immutable binding or immutable `static` are immutable, unless those bytes are part of an [`UnsafeCell<U>`].
 
   Moreover, the bytes [pointed to] by a shared reference, including transitively through other references (both shared and mutable) and `Box`es, are immutable; transitivity includes those references stored in fields of compound types.
 

src/items/external-blocks.md

@@ -56,6 +56,8 @@ of initializing the static.
 Extern statics can be either immutable or mutable just like [statics] outside of external blocks.
 An immutable static *must* be initialized before any Rust code is executed. It is not enough for
 the static to be initialized before Rust code reads from it.
+One Rust code runs, mutating an immutable static (from inside or outside Rust) is UB,
+except if the mutation happens inside an `UnsafeCell`.
 
 ## ABI