add rule on black box/code patching

Author: folkertdev

src/inline-assembly.md

@@ -735,6 +735,11 @@ r[asm.naked-rules.mem-same-as-ffi]
   - Refer to the unsafe code guidelines for the exact rules.
   - These rules do not apply to memory which is private to the asm code, such as stack space allocated within the asm block.
 
+r[asm.rules.black-box]
+- The compiler cannot assume that the instructions in the asm are the ones that will actually end up executed.
+  - This effectively means that the compiler must treat the `naked_asm!` as a black box and only take the interface specification into account, not the instructions themselves.
+  - Runtime code patching is allowed, via target-specific mechanisms.
+
 ### Correctness and Validity
 
 r[asm.validity]