Add admonition about asm attribute implementation

The implementation of attributes for asm macro template strings and
operands doesn't use the normal system in `rustc` for handling
attributes.  This leads to the limitations and may lead to subtle
divergences in behavior.  Let's make a note about this.

For background, see:

- #2063 (comment)
- rust-lang/rust#147736 (comment)

Author: traviscross

src/inline-assembly.md

@@ -289,6 +289,9 @@ core::arch::global_asm!(
 );
 ```
 
+> [!NOTE]
+> In `rustc`, the assembly macros implement handling of these attributes separately from the normal system that handles similar attributes in the language. This accounts for the limited kinds of attributes supported and may give rise to subtle differences in behavior.
+
 r[asm.attributes.starts-with-template]
 Syntactically there must be at least one template string before the first operand.