Callback when the buffer is cleared

[filippo-biondi]

When using nabla.nvim to render latex formulas and entering one of the modes not present in render_modes, some parts of the formulas (those that extend in the lines above or below) are still rendered.

Using this callback, it is possible to fully disable the rendering latex formulas using require('nabla').disable_virt().
I've updated the part of the documentation relative to nabla integration to show this use case.

[MeanderingProgrammer]

Looks good, thanks!