Brief guidance on using CachingOptimizer in solver wrappers (#396)

Ref #395

Author: mlubin

docs/src/apimanual.md

@@ -552,9 +552,9 @@ will additionally support `ScalarAffineFunction`-in-`Interval`.
 
 [The interface is designed for multiple dispatch, e.g., attributes, combinations of sets and functions.]
 
-[Avoid storing extra copies of the problem when possible.]
-
-[`copy!`]
+Avoid storing extra copies of the problem when possible. This means that solver wrappers should not use
+`CachingOptimizer` as part of the wrapper. Instead, just implement `copy!` if the solver's API
+does not support an `addvariable!`-like API. Let users or JuMP decide to use `CachingOptimizer` instead.
 
 ### JuMP mapping