A useful distinction
An introductory message can disappear before the player remembers every rule. Reaching instructions later should be a normal part of the interface. A help screen is especially useful when controls or goals change between modes.
Put it into practice
Test how to open and close help during a session. Check whether the game pauses and whether the board remains intact, then describe that behaviour. Keep headings and navigation clear enough that the player can find the needed rule without rereading unrelated introductory text.