Add warning to check_allocs(...)

This function is useful for investigation and optimization, but we don't
have sufficient parity/stability guarantees from the Julia compiler to
use it for allocation-free guarantees at runtime.

This adds a note to clarify that such a usage is _unsound_.

src/AllocCheck.jl

@@ -170,7 +170,18 @@ end
     check_allocs(func, types; ignore_throw=true)
 
 Compiles the given function and types to LLVM IR and checks for allocations.
-Returns a vector of `AllocationSite` structs, each containing a `CallInst` and a backtrace.
+
+Returns a vector of `AllocationSite`, `DynamicDispatch`, and `AllocatingRuntimeCall`
+
+!!! warning
+    The Julia language/compiler does not guarantee that this result is stable across
+    Julia invocations.
+
+    If you rely on allocation-free code for safety/correctness, it is not sufficient
+    to verify `check_allocs` in test code and expect that the corresponding call in
+    production will not allocate at runtime.
+
+    For this case, you must use `@check_allocs` instead.
 
 # Example
 ```jldoctest