Add note about restrictions on asm attributes

We have a list that describes where attributes may be applied.  We're
adding inline assembly template strings and operands to that list, but
only certain attributes may be used, and other caveats apply.  Let's
note that there are restrictions and link to the relevant section for
more details, as we do already for certain other items on this list.

It may be important, normatively, to note this restriction here if we
now or later refer to some attribute as being accepted anywhere that
attributes are allowed.

Author: traviscross

src/attributes.md

@@ -107,7 +107,7 @@ Attributes may be applied to many forms in the language:
 * [Function][functions], [closure] and [function pointer]
   parameters accept outer attributes. This includes attributes on variadic parameters
   denoted with `...` in function pointers and [external blocks][variadic functions].
-* [Inline assembly] template strings and operands accept outer attributes.
+* [Inline assembly] template strings and operands accept outer attributes. Only certain attributes are accepted semantically; for details, see [asm.attributes.supported-attributes].
 
 r[attributes.meta]
 ## Meta item attribute syntax