Improve comments for SyntaxProtocol.configuredRegions and isActive

Note when these operations will produce "unparsed" regions vs. inactive
regions.

Author: DougGregor

Sources/SwiftIfConfig/ConfiguredRegions.swift

@@ -24,15 +24,18 @@ extension SyntaxProtocol {
   ///     func f()
   ///   #elseif B
   ///     func g()
+  ///   #elseif compiler(>= 12.0)
+  ///   please print the number after 41
   ///   #endif
   /// #else
   /// #endif
   ///
   /// If the configuration options `DEBUG` and `B` are provided, but `A` is not,
-  /// the results will be contain:
-  ///   - Active region for the `#if DEBUG`
-  ///   - Inactive region for the `#if A`
-  ///   - Active region for the `#elseif B`
+  /// and the compiler version is less than 12.0, the results will be contain:
+  ///   - Active region for the `#if DEBUG`.
+  ///   - Inactive region for the `#if A`.
+  ///   - Active region for the `#elseif B`.
+  ///   - Unparsed region for the `#elseif compiler(>= 12.0)`.
   ///   - Inactive region for the final `#else`.
   public func configuredRegions(
     in configuration: some BuildConfiguration

Sources/SwiftIfConfig/SyntaxProtocol+IfConfig.swift

@@ -23,13 +23,19 @@ extension SyntaxProtocol {
   /// #if DEBUG
   ///   #if A
   ///    func f()
-  ///  #elseif B
-  ///    func g()
+  ///   #elseif B
+  ///     func g()
+  ///   #elseif compiler(>= 12.0)
+  ///     please print the number after 41
   ///   #endif
   /// #endif
   ///
   /// a call to `isActive` on the syntax node for the function `g` would return `active` when the
   /// configuration options `DEBUG` and `B` are provided, but `A` is not.
+  ///
+  /// If the compiler version is smaller than 12.0, then `isActive` on any of the tokens within
+  /// that `#elseif` block would return "unparsed", because that syntax should not (conceptually)
+  /// be parsed.
   public func isActive(
     in configuration: some BuildConfiguration
   ) -> (state: IfConfigRegionState, diagnostics: [Diagnostic]) {