Rollup merge of #77295 - jethrogb:jb/unstable-book-asm, r=Amanieu

Update unstable-book: Fix ABNF in inline assembly docs

r? @Amanieu

Author: RalfJung

src/doc/unstable-book/src/library-features/asm.md

@@ -410,7 +410,7 @@ reg_spec := <register class> / "<explicit register>"
 operand_expr := expr / "_" / expr "=>" expr / expr "=>" "_"
 reg_operand := dir_spec "(" reg_spec ")" operand_expr
 operand := reg_operand / "const" const_expr / "sym" path
-option := "pure" / "nomem" / "readonly" / "preserves_flags" / "noreturn" / "att_syntax"
+option := "pure" / "nomem" / "readonly" / "preserves_flags" / "noreturn" / "nostack" / "att_syntax"
 options := "options(" option *["," option] [","] ")"
 asm := "asm!(" format_string *("," format_string) *("," [ident "="] operand) ["," options] [","] ")"
 ```