Mention possible future rejections

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

Author: syvb

src/doc/unstable-book/src/library-features/asm.md

@@ -886,6 +886,6 @@ The compiler performs some additional checks on options:
     - The set of memory locations that you may access is the intersection of those allowed by the `asm!` blocks you entered and exited.
 - You cannot assume that an `asm!` block will appear exactly once in the output binary. The compiler is allowed to instantiate multiple copies of the `asm!` block, for example when the function containing it is inlined in multiple places.
 - On x86, inline assembly must not end with an instruction prefix (such as `LOCK`) that would apply to instructions generated by the compiler.
-  - The compiler is currently unable to detect this due to the way inline assembly is compiled.
+  - The compiler is currently unable to detect this due to the way inline assembly is compiled, but may catch and reject this in the future.
 
 > **Note**: As a general rule, the flags covered by `preserves_flags` are those which are *not* preserved when performing a function call.