Improve the description of axis labels in the "frames" tutorial

[seisman]

Description of the problem

In the frames tutorial, the description of "axis labels" is:

Axis labels will be displayed on all primary axes, which the default is all sides of the figure. To designate only some of the axes as primary, an argument that capitlizes only the primary axes can be passed, which is "WSne" in the example below. The letters correspond with west (left), south (bottom), north (top), and east (right) sides of a figure.

The description about primary axis/ticks/annotations is incorrect and should be fixed.

Please refer to the official description https://docs.generic-mapping-tools.org/latest/cookbook/options.html#map-frame-and-axes-annotations-the-b-option for details.