Document the question mark operator in reference and the book's syntax index

src/doc/book/syntax-index.md

@@ -94,6 +94,7 @@
 * `|=` (`var |= expr`): bitwise or & assignment. Overloadable (`BitOrAssign`).
 * `||` (`expr || expr`): logical or.
 * `_`: "ignored" pattern binding (see [Patterns (Ignoring bindings)]). Also used to make integer-literals readable (see [Reference (Integer literals)]).
+* `?` (`expr?`): Error propagation. Returns early when `Err(_)` is encountered, unwraps otherwise. Similar to the [`try!` macro].
 
 ## Other Syntax
 
@@ -210,6 +211,7 @@
 [Functions]: functions.html
 [Generics]: generics.html
 [Iterators]: iterators.html
+[`try!` macro]: error-handling.html#the-try-macro
 [Lifetimes]: lifetimes.html
 [Loops (`for`)]: loops.html#for
 [Loops (`loop`)]: loops.html#loop

src/doc/reference.md

@@ -2860,8 +2860,8 @@ assert_eq!(x, y);
 
 ### Unary operator expressions
 
-Rust defines the following unary operators. They are all written as prefix operators,
-before the expression they apply to.
+Rust defines the following unary operators. With the exception of `?`, they are
+all written as prefix operators, before the expression they apply to.
 
 * `-`
  : Negation. Signed integer types and floating-point types support negation. It
@@ -2890,6 +2890,10 @@ before the expression they apply to.
  If the `&` or `&mut` operators are applied to an rvalue, a
  temporary value is created; the lifetime of this temporary value
  is defined by [syntactic rules](#temporary-lifetimes).
+* `?`
+ : Propagating errors if applied to `Err(_)` and unwrapping if
+ applied to `Ok(_)`. Only works on the `Result<T, E>` type,
+ and written in postfix notation.
 
 ### Binary operator expressions