Rollup merge of rust-lang#74523 - sollyucko:patch-1, r=Mark-Simulacrum

Improve documentation for `core::fmt` internals

The public interface of `core::fmt` is well-documented, but the internals have very minimal documentation.

src/libcore/fmt/rt/v1.rs

@@ -33,9 +33,13 @@ pub enum Alignment {
     Unknown,
 }
 
+/// Used by [width](https://doc.rust-lang.org/std/fmt/#width) and [precision](https://doc.rust-lang.org/std/fmt/#precision) specifiers.
 #[derive(Copy, Clone)]
 pub enum Count {
+    /// Specified with a literal number, stores the value
     Is(usize),
+    /// Specified using `$` and `*` syntaxes, stores the index into `args`
     Param(usize),
+    /// Not specified
     Implied,
 }