std::fmt: Removed reference to Formatter::buf and other private fields

Formatter::buf is not a public field and therefore isn't very helpful in user-
facing documentation. Also, the other public fields of Formatter were made
private during stabilization of std::fmt (4af3494bb0) and can now only be read
via accessor methods.

library/alloc/src/fmt.rs

@@ -369,9 +369,9 @@
 //! ```
 //!
 //! Your type will be passed as `self` by-reference, and then the function
-//! should emit output into the `f.buf` stream. It is up to each format trait
-//! implementation to correctly adhere to the requested formatting parameters.
-//! The values of these parameters will be listed in the fields of the
+//! should emit output into the Formatter `f` which implements `fmt::Write`. It is up to each
+//! format trait implementation to correctly adhere to the requested formatting parameters.
+//! The values of these parameters can be accessed with methods of the
 //! [`Formatter`] struct. In order to help with this, the [`Formatter`] struct also
 //! provides some helper methods.
 //!