Fix the doc formatting for FloatingPoint comparison methods (swiftlang#78443)

In the documentation for the `isLess(than:)` and `isLessThanOrEqualTo(_:)`
methods, a code sample isn't being formatted correctly due to it directly
following an unordered list. This change adds an additional message that
introduces the code sample, separating it from the list and allowing
the correct formatting to be applied.

Author: natecook1000

stdlib/public/core/FloatingPoint.swift

@@ -1048,6 +1048,9 @@ public protocol FloatingPoint: SignedNumeric, Strideable, Hashable
   /// - Every value except for NaN and `+infinity` compares less than
   ///   `+infinity`.
   ///
+  /// The following example shows the behavior of the `isLess(than:)` method
+  /// with different kinds of values:
+  ///
   ///     let x = 15.0
   ///     x.isLess(than: 20.0)
   ///     // true
@@ -1078,6 +1081,9 @@ public protocol FloatingPoint: SignedNumeric, Strideable, Hashable
   /// - `-infinity` compares less than or equal to all values except NaN.
   /// - Every value except NaN compares less than or equal to `+infinity`.
   ///
+  /// The following example shows the behavior of the `isLessThanOrEqualTo(_:)`
+  /// method with different kinds of values:
+  ///
   ///     let x = 15.0
   ///     x.isLessThanOrEqualTo(20.0)
   ///     // true