docs: document the ability to save parameters in callbacks

Author: AayushSabharwal

docs/src/basics/Events.md

@@ -336,3 +336,45 @@ one must still use a vector
 ```julia
 discrete_events = [[2.0] => [v ~ -v]]
 ```
+
+## Saving discrete values
+
+Time-dependent parameters which are updated in callbacks are termed as discrete variables.
+ModelingToolkit enables automatically saving the timeseries of these discrete variables,
+and indexing the solution object to obtain the saved timeseries. Consider the following
+example:
+
+```@example events
+@variables x(t)
+@parameters c(t)
+
+@mtkbuild sys = ODESystem(
+    D(x) ~ c * cos(x), t, [x], [c]; discrete_events = [1.0 => [c ~ c + 1]])
+
+prob = ODEProblem(sys, [x => 0.0], (0.0, 2pi), [c => 1.0])
+sol = solve(prob, Tsit5())
+sol[c]
+```
+
+The solution object can also be interpolated with the discrete variables
+
+```@example events
+sol([1.0, 2.0], idxs = [c, c * cos(x)])
+```
+
+Note that only time-dependent parameters will be saved. If we repeat the above example with
+this change:
+
+```@example events
+@variables x(t)
+@parameters c
+
+@mtkbuild sys = ODESystem(
+    D(x) ~ c * cos(x), t, [x], [c]; discrete_events = [1.0 => [c ~ c + 1]])
+
+prob = ODEProblem(sys, [x => 0.0], (0.0, 2pi), [c => 1.0])
+sol = solve(prob, Tsit5())
+sol.ps[c] # sol[c] will error, since `c` is not a timeseries value
+```
+
+It can be seen that the timeseries for `c` is not saved.