Clarify interaction of asm-goto with IBT

Author: nbdd0121

src/inline-assembly.md

@@ -357,6 +357,7 @@ assert_eq!(y, [3, 2, 0, 1]);
 r[asm.operand-type.supported-operands.label]
 * `label <block>`
   - The address of the block is substituted into the asm template string. The assembly code may jump to the substituted address.
+  - For targets that distinguish between direct jumps and indirect jumps (e.g. x86-64 with `cf-protection` enabled), the assembly code must not jump to the substituted address indirectly.
   - After execution of the block, the `asm!` expression returns.
   - The type of the block must be unit or `!` (never).
   - The block starts a new safety context; unsafe operations within the `label` block must be wrapped in an inner `unsafe` block, even though the entire `asm!` expression is already wrapped in `unsafe`.