Document map key and value fields (#603)

protobuf/lib/src/protobuf/field_info.dart

diff --git a/protobuf/lib/src/protobuf/field_info.dart b/protobuf/lib/src/protobuf/field_info.dart
index a825f6a..2d668cf 100644
--- a/protobuf/lib/src/protobuf/field_info.dart
+++ b/protobuf/lib/src/protobuf/field_info.dart
@@ -221,7 +221,16 @@
 }
 
 class MapFieldInfo<K, V> extends FieldInfo<PbMap<K, V>?> {
+  /// Key type of the map. Per proto2 and proto3 specs, this needs to be an
+  /// integer type or `string`, and the type cannot be `repeated`.
+  ///
+  /// The `int` value is interpreted the same way as [FieldInfo.type].
   final int? keyFieldType;
+
+  /// Value type of the map. Per proto2 and proto3 specs, this can be any type
+  /// other than `map`, and the type cannot be `repeated`.
+  ///
+  /// The `int` value is interpreted the same way as [FieldInfo.type].
   final int? valueFieldType;
 
   /// Creates a new empty instance of the value type.

protobuf/lib/src/protobuf/pb_map.dart

diff --git a/protobuf/lib/src/protobuf/pb_map.dart b/protobuf/lib/src/protobuf/pb_map.dart
index 5c2c28c..7449856 100644
--- a/protobuf/lib/src/protobuf/pb_map.dart
+++ b/protobuf/lib/src/protobuf/pb_map.dart
@@ -5,7 +5,16 @@
 part of protobuf;
 
 class PbMap<K, V> extends MapBase<K, V> {
+  /// Key type of the map. Per proto2 and proto3 specs, this needs to be an
+  /// integer type or `string`, and the type cannot be `repeated`.
+  ///
+  /// The `int` value is interpreted the same way as [FieldInfo.type].
   final int? keyFieldType;
+
+  /// Value type of the map. Per proto2 and proto3 specs, this can be any type
+  /// other than `map`, and the type cannot be `repeated`.
+  ///
+  /// The `int` value is interpreted the same way as [FieldInfo.type].
   final int? valueFieldType;
 
   static const int _keyFieldNumber = 1;