Outputs no longer have to be mutable

Author: ubsan

src/doc/trpl/inline-assembly.md

@@ -77,7 +77,7 @@ asm!("xor %eax, %eax" ::: "{eax}");
 
 Input and output operands follow the same format: `:
 "constraints1"(expr1), "constraints2"(expr2), ..."`. Output operand
-expressions must be mutable lvalues:
+expressions must be mutable lvalues, or not yet assigned:
 
 ```
 # #![feature(asm)]