Explain in theory docs how `calculate_limit_axial_load` works

[talledodiego]

Add in the theory documentation a paragraph or a section explaining this aspect.

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