Document lifetime elision for fn types, Fn*, impl

Currently, the lifetime elision doc only documents function definitions, but lifetime elision is also allowed in the following other locations:

* `fn` types, such as `fn(&T)`
* `Fn`/`FnMut`/`FnOnce`, such as `Fn(&T)`
* `impl` headers

To demo this up, I made some type aliases for `fn`/`Fn` which you can pass `&T` as a parameter to (to follow the lifetime rules of the surrounding context), and compared what you get with that instead of using `fn`/`Fn` directly, where lifetime elision takes on the rules from `fn`/`Fn`/etc.

I also demoed up an `impl` header that used lifetime elision twice, although the error message in that case is broken (filed rust-lang/rust#87763)

The demo was half for this change description, and half just to make sure I understand Rust -- in particular, I really had to reverse engineer it for `impl` because I wasn't sure, and it didn't seem to be documented anywhere (at least not here!)

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f82b280de4b992f225bc32121f333e96

Author: ssbr

src/lifetime-elision.md

@@ -14,15 +14,17 @@ T<'a>
 
 Lifetime positions can appear as either "input" or "output":
 
-* For `fn` definitions, input refers to the types of the formal arguments
-  in the `fn` definition, while output refers to
+* For `fn` definitions, `fn` types, and the traits `Fn`, `FnMut`, and `FnOnce`,
+  input refers to the types of the formal arguments, while output refers to
   result types. So `fn foo(s: &str) -> (&str, &str)` has elided one lifetime in
-  input position and two lifetimes in output position.
-  Note that the input positions of a `fn` method definition do not
-  include the lifetimes that occur in the method's `impl` header
-  (nor lifetimes that occur in the trait header, for a default method).
-
-* In the future, it should be possible to elide `impl` headers in the same manner.
+  input position and two lifetimes in output position. Note that the input
+  positions of a `fn` method definition do not include the lifetimes that occur
+  in the method's `impl` header (nor lifetimes that occur in the trait header,
+  for a default method).
+
+* For `impl` headers, all types are input. So `impl Trait<&T> for Struct<&T>`
+  has elided two lifetimes in input position, while `impl Struct<&T>` has elided
+  one.
 
 Elision rules are as follows: