Clarify that format string implicit named arguments are not supported

Author: Amanieu

rfcs/0000-inline-asm.md

@@ -317,7 +317,7 @@ asm := "asm!(" format_string *("," [ident "="] operand) ["," flags] ")"
 
 ## Template string
 
-The assembler template uses the same syntax as [format strings][format-syntax] (i.e. placeholders are specified by curly braces). The corresponding arguments are accessed in order, by index, or by name.
+The assembler template uses the same syntax as [format strings][format-syntax] (i.e. placeholders are specified by curly braces). The corresponding arguments are accessed in order, by index, or by name. However, implicit named arguments (introduced by [RFC #2795][rfc-2795]) are not supported.
 
 The assembly code syntax used is that of the GNU assembler (GAS). The only exception is on x86 where the Intel syntax is used instead of GCC's AT&T syntax.
 
@@ -327,6 +327,8 @@ However there is one restriction on the asm string: any assembler state (e.g. th
 
 The compiler will lint against any operands that are not used in the template string, except for operands that specify an explicit register.
 
+[rfc-2795]: https://github.com/rust-lang/rfcs/pull/2795
+
 ## Operand type
 
 Several types of operands are supported: