docs: update rationale for excessive-bools (#14351)

Adds the reasoning to the docs for this rule on why enums are generally
better for representing state machines than structs with many bool
fields.

changelog: [struct_excessive_bools]: Improve documentation

Author: blyxyas

clippy_lints/src/excessive_bools.rs

@@ -15,12 +15,17 @@ declare_clippy_lint! {
     /// use of bools in structs.
     ///
     /// ### Why is this bad?
-    /// Excessive bools in a struct
-    /// is often a sign that it's used as a state machine,
-    /// which is much better implemented as an enum.
-    /// If it's not the case, excessive bools usually benefit
-    /// from refactoring into two-variant enums for better
-    /// readability and API.
+    /// Excessive bools in a struct is often a sign that
+    /// the type is being used to represent a state
+    /// machine, which is much better implemented as an
+    /// enum.
+    ///
+    /// The reason an enum is better for state machines
+    /// over structs is that enums more easily forbid
+    /// invalid states.
+    ///
+    /// Structs with too many booleans may benefit from refactoring
+    /// into multi variant enums for better readability and API.
     ///
     /// ### Example
     /// ```no_run