rustdoc: update book with info on type bindings

src/doc/rustdoc/src/read-documentation/search.md

@@ -72,6 +72,7 @@ the standard library and functions that are included in the results list:
 | [`stdout, [u8]`][stdoutu8] | `Stdout::write` |
 | [`any -> !`][] | `panic::panic_any` |
 | [`vec::intoiter<T> -> [T]`][iterasslice] | `IntoIter::as_slice` and `IntoIter::next_chunk` |
+| [`iterator<T>, fnmut -> T`][iterreduce] | `Iterator::reduce` and `Iterator::find` |
 
 [`usize -> vec`]: ../../std/vec/struct.Vec.html?search=usize%20-%3E%20vec&filter-crate=std
 [`vec, vec -> bool`]: ../../std/vec/struct.Vec.html?search=vec,%20vec%20-%3E%20bool&filter-crate=std
@@ -81,6 +82,7 @@ the standard library and functions that are included in the results list:
 [`any -> !`]: ../../std/vec/struct.Vec.html?search=any%20-%3E%20!&filter-crate=std
 [stdoutu8]: ../../std/vec/struct.Vec.html?search=stdout%2C%20[u8]&filter-crate=std
 [iterasslice]: ../../std/vec/struct.Vec.html?search=vec%3A%3Aintoiter<T>%20->%20[T]&filter-crate=std
+[iterreduce]: ../../std/index.html?search=iterator<T>%2C%20fnmut%20->%20T&filter-crate=std
 
 ### How type-based search works
 
@@ -95,7 +97,9 @@ After deciding which items are type parameters and which are actual types, it
 then searches by matching up the function parameters (written before the `->`)
 and the return types (written after the `->`). Type matching is order-agnostic,
 and allows items to be left out of the query, but items that are present in the
-query must be present in the function for it to match.
+query must be present in the function for it to match. The `self` parameter is
+treated the same as any other parameter, and `Self` is resolved to the
+underlying type's name.
 
 Function signature searches can query generics, wrapped in angle brackets, and
 traits will be normalized like types in the search engine if no type parameters
@@ -103,8 +107,37 @@ match them. For example, a function with the signature
 `fn my_function<I: Iterator<Item=u32>>(input: I) -> usize`
 can be matched with the following queries:
 
-* `Iterator<u32> -> usize`
+* `Iterator<Item=u32> -> usize`
-* `Iterator -> usize`
+* `Iterator<u32> -> usize` (you can leave out the `Item=` part)
+* `Iterator -> usize` (you can leave out iterator's generic entirely)
+* `T -> usize` (you can match with a generic parameter)
+
+Each of the above queries is progressively looser, except the last one
+would not match `dyn Iterator`, since that's not a type parameter.
+
+If a bound has multiple associated types, specifying the name allows you to
+pick which one gets matched. If no name is specified, then the query will
+match of any of them. For example,
+
+```rust
+pub trait MyTrait {
+    type First;
+    type Second;
+}
+
+/// This function can be found using the following search queries:
+///
+///     MyTrait<First=u8, Second=u32> -> bool
+///     MyTrait<u32, First=u8> -> bool
+///     MyTrait<Second=u32> -> bool
+///     MyTrait<u32, u8> -> bool
+///
+/// The following queries, however, will *not* match it:
+///
+///     MyTrait<First=u32> -> bool
+///     MyTrait<u32, u32> -> bool
+pub fn my_fn(x: impl MyTrait<First=u8, Second=u32>) -> bool { true }
+```
 
 Generics and function parameters are order-agnostic, but sensitive to nesting
 and number of matches. For example, a function with the signature
@@ -134,6 +167,10 @@ Most of these limitations should be addressed in future version of Rustdoc.
     with that bound, it'll match, but `option<T> -> T where T: Default`
     cannot be precisely searched for (use `option<Default> -> Default`).
 
+  * Supertraits, type aliases, and Deref are all ignored. Search mostly
+    operates on type signatures *as written*, and not as they are
+    represented within the compiler.
+
   * Type parameters match type parameters, such that `Option<A>` matches
     `Option<T>`, but never match concrete types in function signatures.
     A trait named as if it were a type, such as `Option<Read>`, will match
@@ -183,7 +220,8 @@ slice = OPEN-SQUARE-BRACKET [ nonempty-arg-list ] CLOSE-SQUARE-BRACKET
 arg = [type-filter *WS COLON *WS] (path [generics] / slice / [!])
 type-sep = COMMA/WS *(COMMA/WS)
 nonempty-arg-list = *(type-sep) arg *(type-sep arg) *(type-sep)
-generics = OPEN-ANGLE-BRACKET [ nonempty-arg-list ] *(type-sep)
+generic-arg-list = *(type-sep) arg [ EQUAL arg ] *(type-sep arg [ EQUAL arg ]) *(type-sep)
+generics = OPEN-ANGLE-BRACKET [ generic-arg-list ] *(type-sep)
             CLOSE-ANGLE-BRACKET
 return-args = RETURN-ARROW *(type-sep) nonempty-arg-list
 
@@ -230,6 +268,7 @@ DOUBLE-COLON = "::"
 QUOTE = %x22
 COMMA = ","
 RETURN-ARROW = "->"
+EQUAL = "="
 
 ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
 DIGIT = %x30-39