Fix docs for coastline alteration

[xylar]

They were using the old (C++ based) mask creator but need to be updated to use functions in mpas_tools.mesh.mask now that the mpas_tools.mesh.conversion.mask() wrapper function uses the newer python-based mask creation and only creates region masks, not transect masks.

[xylar]

@vanroekel, please confirm that the updated documentation now works for you, following up on our conversation on Slack.

[xylar]

Thanks @vanroekel!