Document parameters given to store_tokens callback

### Is your feature request related to a problem? Please describe.

I couldn't find this documented anywhere and had to dig into the source to find its arguments

### Describe the solution you'd like

Document it in the relevant docstrings and the mentions in the README

### Describe alternatives you've considered

n/a

### Additional context

n/a

Locate the store_tokens callback definition and read its current docstring and the README mentions of the callback. Document the callback parameters consistently in those locations, then verify that the documented arguments match the source implementation.

Written by the indexing model from the issue text.