flip does not document the mainAxis and altAxis options

[dilyanpalauzov]

#1095 adds the mainAxis and altAxis options to the flip modifier.

https://popper.js.org/docs/v2/modifiers/flip/ does not explain what are these options good for.

[FezVrasta]

@atomiks correct me if I'm wrong, but I think these two options were only available on Popper 1.x, they were removed in 2.x

[atomiks]

These are v2.x only, so the documentation is missing them

[atomiks]

Can you make this issue at https://github.com/floating-ui/popper.js.org? For documentation, issues in this repo should now pertain to Floating UI. Thanks!