TriaxialStressController (doc) inconsistent numbering of axes

In TriaxialStressController (and current master)

• width, depth, height or wall_*_id number axes from 0 to 2
• while goal* and max_vel* use a numbering from 1 to 3

This makes things a bit weird, where 1-axis in height does not refer to the same thing as axis 1 in goal1

Outside of the possibility of replacing goal1 to goal3 with sthg like goal0 to goal2 (for which there may be never consensus ?), probably we should at least update the doc ?