feat: Add documentation for generated service constructors (#2575)

Signed-off-by: Brian Quinlan <[email protected]>
Co-authored-by: Nate Bosch <[email protected]>

Author: brianquinlan

internal/sidekick/internal/dart/templates/lib/service.mustache

@@ -21,9 +21,27 @@ final class {{Codec.Name}} {
   static const _host = '{{DefaultHost}}';
   final ServiceClient _client;
 
+  /// Creates a `{{Codec.Name}}` using [client] for transport.
+  ///
+  /// The provided [http.Client] must be configured to provide whatever
+  /// authentication is required by `{{Codec.Name}}`. You can do that
+  /// using
+  /// [`package:googleapis_auth`](https://pub.dev/packages/googleapis_auth).
   {{Codec.Name}}({required http.Client client})
       : _client = ServiceClient(client: client);
 
+  /// Creates a `{{Codec.Name}}` that does authentication through an API key.
+  ///
+  /// If called without arguments, the API key is taken from these environment
+  /// variables:
+{{#Model.Codec.ApiKeyEnvironmentVariables}}
+  ///   * `{{{.}}}`
+{{/Model.Codec.ApiKeyEnvironmentVariables}}
+  ///
+  /// Throws [ArgumentError] if called without arguments and none of the above
+  /// environment variables are set.
+  ///
+  /// See [API Keys Overview](https://cloud.google.com/api-keys/docs/overview).
   factory {{Codec.Name}}.fromApiKey([String? apiKey]) {
     apiKey ??= _apiKeys.map(environmentVariable).nonNulls.firstOrNull;
     if (apiKey == null) {