Update docs per reviewers

Signed-off-by: Michael Carroll <michael@openrobotics.org>

rmw/include/rmw/message_sequence.h

@@ -85,17 +85,33 @@ RMW_PUBLIC
 rmw_ret_t
 rmw_message_sequence_fini(rmw_message_sequence_t * sequence);
 
+/// Return an rmw_message_info_sequence_t struct with members initialized to `NULL`
 RMW_PUBLIC
 rmw_message_info_sequence_t
 rmw_get_zero_initialized_message_info_sequence(void);
 
+/// Initialize an rmw_message_info_sequence_t object.
+/**
+ * \param[inout] sequence sequence object to be initialized.
+ * \param[in] size capacity of the sequence to be allocated.
+ * \param[in] allocator the allcator used to allocate memory.
+ */
 RMW_PUBLIC
 rmw_ret_t
 rmw_message_info_sequence_init(
   rmw_message_info_sequence_t * sequence,
   size_t size,
   rcutils_allocator_t * allocator);
 
+/// Finalize an rmw_message_sequence_t object.
+/**
+ * The rmw_message_sequence_t struct has members which require memory to be allocated to them
+ * before setting values.
+ * This function reclaims any allocated resources within the object and zeroes out all other
+ * members.
+ *
+ * \param[inout] sequence sequence object to be finalized.
+ */
 RMW_PUBLIC
 rmw_ret_t
 rmw_message_info_sequence_fini(rmw_message_info_sequence_t * sequence);