Document fully-qualified syntax in as' keyword doc

Author: fpdotmonkey

library/std/src/keyword_docs.rs

@@ -24,6 +24,13 @@
 /// (`u8`, `bool`, `str`, pointers, ...) whereas `From` and `Into`  also works with types like
 /// `String` or `Vec`.
 ///
+/// You'll also find with `From` and `Into`, and indeed all traits, that `as` is used for the
+/// _fully qualified syntax_, a means of clarifying ambiguous method calls.  If you have a type which
+/// implements two traits with identical method names (e.g. `Into<u32>` and `Into<u64>`), you can
+/// clarify which method you use with `<MyThing as Into<u32>>::into(my_thing)`.  This is quite
+/// verbose, but fortunately, Rust's type inference usually saves you from needing this, although it
+/// is occasionally necessary.
+///
 /// `as` can also be used with the `_` placeholder when the destination type can be inferred. Note
 /// that this can cause inference breakage and usually such code should use an explicit type for
 /// both clarity and stability. This is most useful when converting pointers using `as *const _` or