Doc: Remove text about default semantic action for _terminal.
Doc: Remove text about default semantic action for _terminal.

As of 22ff905, there is no longer a default action for terminal nodes.
coopsource/ohm,harc/ohm,harc/ohm,coopsource/ohm,cdglabs/ohm,cdglabs/ohm,cdglabs/ohm,harc/ohm,coopsource/ohm