Provide help either on a specific command (if a search string is provided and an exact match is found) or on the available commands (if a search string is not provided, or if no exact match is found).
Darparu cymorth un ai ar orchymyn penodol (os oes llinyn chwilio'n cael ei darparu ac mae cytunedd i'w weld) neu ar orchmynion sydd ar gael (os nad oes llinyn chwilio'n cael ei ddarparu, neu os nad oes cytunedd union).