Add example to cols_label_rotate of 3 dir styles used side-by-side

[machow]

This PR #696 added a .cols_label_rotate() method that supports three directions for rotating. Let's add an example to the docstring of the three directions shown side by side (e.g. a frame with 3 columns, 1 for each, the column names could be the directions or something similar).

[AdrienDart]

Hi, following up on that, is there an option to specify some degree of orientation too? (Rotate text by 30 degrees for example) Thanks