bug#33664: 26.1; Document vars and functions in `cursor-sensor.el'

[Juri Linkov]

> Juri reads bug reports, and he has worked on Isearch
> (dunno whether he worked on this part of it).  Maybe
> the design/use of the var for Isearch could be
> documented in a comment in `isearch.el', at least.

I could help but I know nothing about cursor-sensor.
Please explain the purpose of cursor-sensor, then
I'll write a comment in `isearch.el'.