[doc] update doc-string in axis

Merged Linus Pithan requested to merge update_axis_doc_str into master

I took some of the changes made by Linus to avoid a fastidious rebase. + updated the documentation doc.

LINUS: I tried to update doc-strings in axis object to be compliant with


@cyril.guilloud , @pancino maybe you want to have a look if you agree with this format.

Edited by Cyril Guilloud

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