docs: document inverse function registration

docs/src/manual/functions.md

@@ -188,3 +188,21 @@ function Symbolics.derivative(interp::AbstractInterpolation, args::NTuple{1, Any
     Symbolics.unwrap(derivative(interp, Symbolics.wrap(args[1])))
 end
 ```
+
+## Inverse function registration
+
+Symbolics.jl allows defining and querying the inverses of functions.
+
+```@docs
+inverse
+left_inverse
+right_inverse
+@register_inverse
+has_inverse
+has_left_inverse
+has_right_inverse
+```
+
+Symbolics.jl implements inverses for standard trigonometric and logarithmic functions,
+as well as their variants from `NaNMath`. It also implements inverses of
+`ComposedFunction`s.