This command is not necessarily specific to MST. This is a generic spanning-tree command which can be used in all modes.
As Reza explained, the spanning-tree portfast trunk, enables the portfast feature on a trunking interface - configuring just spanning-tree portfast on a trunking interface has no effect. This is most commonly used when you have trunked interfaces that go to servers, etc.
The spanning-tree portfast default command is an easy way of enabling the portfast feature on all access ports instead of going into each interface and enabling it one by one.