Remove the definition of "safe".

`stack_addr` and unrestricted loads and stores *can* be used with entirely
defined behavior. The sense in which they're not "safe" is only that it's
possible to misuse them. This subtlety wasn't captured in the definition
of "safe" here, so for now, just remove the definition so that it doesn't
cause confusion.

cranelift/docs/langref.rst

@@ -497,8 +497,8 @@ about because stack slots and offsets are fixed at compile time. For example,
 the alignment of these stack memory accesses can be inferred from the offsets
 and stack slot alignments.
 
-It can be necessary to escape from the :term:`safety` of the restricted
-instructions by taking the address of a stack slot.
+It's also possible to obtain the address of a stack slot, which can be used
+in :ref:`unrestricted loads and stores <memory>`.
 
 .. autoinst:: stack_addr
 
@@ -1113,15 +1113,6 @@ Glossary
         intermediate representation. Cretonne's IR can be converted to text
         losslessly.
 
-    safe
-    safety
-        Execution of exclusively defined behavior. Safe programs cannot
-        read, write, or execute memory outside of heaps, globals, stack
-        regions, and functions that have been explicitly provided to them. In
-        some instances, defined behavior can be nondeterministic, where the
-        specific behavior may vary among a bounded set of possibilities.
-        Execution of undefined behavior is unsafe.
-
     stack slot
         A fixed size memory allocation in the current function's activation
         frame. Also called a local variable.