Document how to mutate parameters p in integrator (fixes #243)

Add documentation explaining that parameters can be modified directly
via `integrator.p = new_p`, unlike u and t which require setter functions.

This addresses the user's question about how to change the differential
equation parameters during integration, particularly in callbacks.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

docs/src/basics/integrator.md

@@ -144,6 +144,13 @@ As low-level alternative to the callbacks, one can use `set_t!`, `set_u!` and
 have efficient ways to modify `u` and `t`.  In such case, `set_*!` are as
 inefficient as `reinit!`.
 
+For modifying parameters `p`, you can directly assign to the field:
+```julia
+integrator.p = new_p
+```
+Unlike `u` or `t`, changing `p` doesn't affect the solver's internal state or
+accuracy, so it can be modified directly without a setter function.
+
 ```@docs
 SciMLBase.set_t!
 SciMLBase.set_u!