Improve docstrings to make clear the idea behind `calculate_axial_limit_load`

[talledodiego]

Improve the docstrings to state that axial limit load is intended as uniform compression or tension stresses in the section.

I totally agree with you, and thanks for raising this issue, which allowed us to look further into it.

I suggest the following plan that can be pursued also with separate PRs, if you agree:

• Improve docstrings to make this clear. Possibly in the short term.
• Create a section in the theory docs specifically stating this aspect. Possibility in the short term.
• Eventually consider adding a usage example that shows how to extract the green points. Medium term.
• Eventually consider adding specific methods in the results objects to extract those green points. Medium term.

Originally posted by @talledodiego in #330