add doc for array fields size and capacity

rosidl_generator_c/include/rosidl_generator_c/primitives_array.h

@@ -23,8 +23,8 @@
   typedef struct rosidl_generator_c__ ## STRUCT_NAME ## __Array \
   { \
     TYPE_NAME * data; \
-    size_t size; \
-    size_t capacity; \
+    size_t size; /*!< The number of valid items in data */ \
+    size_t capacity; /*!< The number of allocated items in data */ \
   } rosidl_generator_c__ ## STRUCT_NAME ## __Array;
 
 // array types for all primitive types

rosidl_generator_c/resource/msg__struct.h.em

@@ -165,7 +165,9 @@ typedef struct @(msg_typename)
 typedef struct @(array_typename)
 {
   @(msg_typename) * data;
+  /// The number of valid items in data
   size_t size;
+  /// The number of allocated items in data
   size_t capacity;
 } @(array_typename);