Update unstable-book with recent asm changes (may_unwind)

Author: cynecx

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

@@ -454,7 +454,7 @@ operand_expr := expr / "_" / expr "=>" expr / expr "=>" "_"
 reg_operand := dir_spec "(" reg_spec ")" operand_expr
 operand := reg_operand / "const" const_expr / "sym" path
 clobber_abi := "clobber_abi(" <abi> *["," <abi>] [","] ")"
-option := "pure" / "nomem" / "readonly" / "preserves_flags" / "noreturn" / "nostack" / "att_syntax" / "raw"
+option := "pure" / "nomem" / "readonly" / "preserves_flags" / "noreturn" / "nostack" / "att_syntax" / "raw" / "may_unwind"
 options := "options(" option *["," option] [","] ")"
 asm := "asm!(" format_string *("," format_string) *("," [ident "="] operand) *("," clobber_abi) *("," options) [","] ")"
 ```
@@ -829,6 +829,7 @@ Currently the following options are defined:
 - `noreturn`: The `asm` block never returns, and its return type is defined as `!` (never). Behavior is undefined if execution falls through past the end of the asm code. A `noreturn` asm block behaves just like a function which doesn't return; notably, local variables in scope are not dropped before it is invoked.
 - `nostack`: The `asm` block does not push data to the stack, or write to the stack red-zone (if supported by the target). If this option is *not* used then the stack pointer is guaranteed to be suitably aligned (according to the target ABI) for a function call.
 - `att_syntax`: This option is only valid on x86, and causes the assembler to use the `.att_syntax prefix` mode of the GNU assembler. Register operands are substituted in with a leading `%`.
+- `may_unwind`: The `asm` block may unwind the stack and be part of the stack unwinding process (This option is only supported by the LLVM backend right now).
 - `raw`: This causes the template string to be parsed as a raw assembly string, with no special handling for `{` and `}`. This is primarily useful when including raw assembly code from an external file using `include_str!`.
 
 The compiler performs some additional checks on options: