Add file-level comments

Author: yevhenii-nadtochii

model/src/testFixtures/proto/spine/validation/goes_bad_companion_type.proto

@@ -36,12 +36,10 @@ option java_package = "io.spine.validation";
 option java_outer_classname = "GoesBadCompanionTypeProto";
 option java_multiple_files = true;
 
-/*
-Incompatible field types:
-    1. double, float.
-    2. all integers.
-    3. bool.
- */
+// This file declares messages that specify a field with incompatible type as a companion.
+// Incompatible field types are the following:
+// 1. All numeric fields.
+// 3. Boolean.
 
 message GoesBoolCompanion {
     bool companion = 1;

model/src/testFixtures/proto/spine/validation/goes_bad_target_type.proto

@@ -36,12 +36,10 @@ option java_package = "io.spine.validation";
 option java_outer_classname = "GoesBadTargetTypeProto";
 option java_multiple_files = true;
 
-/*
-Incompatible field types:
-    1. double, float.
-    2. all integers.
-    3. bool.
- */
+// This file declares messages that apply `(goes)` option to fields with incompatible type.
+// Incompatible field types are the following:
+// 1. All numeric fields.
+// 3. Boolean.
 
 message GoesBoolTarget {
     string companion = 1;