Add more comments calling out things in the file.

Since developers might need to go looking into the source for some of the
extension related things, add command/marks into the file to hopefully make it
easier to find the using things.

Author: thomasvl

Sources/protoc-gen-swift/ExtensionSetGenerator.swift

@@ -159,6 +159,15 @@ class ExtensionSetGenerator {
     func generateMessageSwiftExtensions(printer p: inout CodePrinter) {
         guard !extensions.isEmpty else { return }
 
+        p.print(
+            "\n",
+            "// MARK: - Extension Properties\n",
+            "\n",
+            "// Swift Extensions on the exteneded Messages to add easy access to the declared\n",
+            "// extension fields. The names are based on the extension field name from the proto\n",
+            "// declaration. To avoid naming collisions, the names are prefixed with the name of\n",
+            "// the scope where the extend directive occurs.\n")
+
         // Reorder the list so they are grouped by the Message being extended, but
         // maintaining the order they were within the file within those groups.
         let grouped: [ExtensionGenerator] = extensions.enumerated().sorted {
@@ -206,6 +215,8 @@ class ExtensionSetGenerator {
         let filenameAsIdentifer = NamingUtils.toUpperCamelCase(pathParts.base)
         let filePrefix = namer.typePrefix(forFile: fileDescriptor)
         p.print(
+          "\n",
+          "// MARK: - File's ExtensionMap: \(filePrefix)\(filenameAsIdentifer)_Extensions\n",
           "\n",
           "/// A `SwiftProtobuf.SimpleExtensionMap` that includes all of the extensions defined by\n",
           "/// this .proto file. It can be used any place an `SwiftProtobuf.ExtensionMap` is needed\n",
@@ -226,6 +237,12 @@ class ExtensionSetGenerator {
     func generateProtobufExtensionDeclarations(printer p: inout CodePrinter) {
       guard !extensions.isEmpty else { return }
 
+      p.print(
+          "\n",
+          "// Extension Objects - The only reason these might be needed is when manually\n",
+          "// constructing a `SimpleExtensionMap`, otherwise, use the above _Extension Properties_\n",
+          "// accessors for the extension fields on the messages directly.\n")
+
       func endScope() {
           p.outdent()
           p.print("}\n")