MCA/base: document that is is valid for the string_value parameter to

an enumerator's mca_base_var_enum_sfv_fn_t can be NULL.

cmr=v1.7.5:ticket=trac:4398:reviewer=ompi-gk1.7

This commit was SVN r31085.

The following Trac tickets were found above:
  Ticket 4398 --> https://svn.open-mpi.org/trac/ompi/ticket/4398

opal/mca/base/mca_base_var_enum.h

@@ -82,6 +82,10 @@ typedef int (*mca_base_var_enum_dump_fn_t)(mca_base_var_enum_t *self, char **out
  *
  * @retval OPAL_SUCCESS on success
  * @retval OPAL_ERR_VALUE_OUT_OF_BOUNDS if not found
+ *
+ * @long This function returns the string value for a given interger value in the
+ * {string_value} parameter. The {string_value} parameter may be NULL in which case
+ * no string is returned.
  */
 typedef int (*mca_base_var_enum_sfv_fn_t)(mca_base_var_enum_t *self, const int value,
                                           const char **string_value);