Clarify documentation for H5CX_get_data_transform (#4580)

* Correct comment for H5CX_get_data_transform

* Document why data transform ctx field doesnt use macro

src/H5CX.c

@@ -2351,7 +2351,7 @@ H5CX_get_filter_cb(H5Z_cb_t *filter_cb)
 /*-------------------------------------------------------------------------
  * Function: H5CX_get_data_transform
  *
- * Purpose: Retrieves the I/O filter callback function for the current API call context.
+ * Purpose: Retrieves the data transformation expression for the current API call context.
  *
  * Return: Non-negative on success / Negative on failure
  *
@@ -2371,6 +2371,9 @@ H5CX_get_data_transform(H5Z_data_xform_t **data_transform)
  assert(head && *head);
  assert(H5P_DEFAULT != (*head)->ctx.dxpl_id);
 
+ /* This getter does not use H5CX_RETRIEVE_PROP_VALID in order to use H5P_peek instead of H5P_get.
+ This prevents invocation of the data transform property's library-defined copy callback */
+
  /* Check if the value has been retrieved already */
  if (!(*head)->ctx.data_transform_valid) {
  /* Check for default DXPL */