Clarify that modifyproperty! does not call convert (#45178)

base/docs/basedocs.jl

@@ -2758,6 +2758,11 @@ The syntax `@atomic max(a().b, c)` returns `modifyproperty!(a(), :b,
 max, c, :sequentially_consistent))`, where the first argument must be a
 `getfield` expression and is modified atomically.
 
+Unlike [`setproperty!`](@ref Base.setproperty!), the default implementation of
+`modifyproperty!` does not call `convert` automatically.  Thus, `op` must return a value
+that can be stored in the field `f` directly when invoking the default `modifyproperty!`
+implementation.
+
 See also [`modifyfield!`](@ref Core.modifyfield!)
 and [`setproperty!`](@ref Base.setproperty!).
 """