fix caching docs

Author: ChrisRackauckas

docs/src/tutorials/caching_interface.md

@@ -55,7 +55,7 @@ Then refactorization will occur when a new `A` is given:
 
 ```julia
 A2 = rand(n,n)
-linsolve = LinearSolve.set_A(linsolve,A2)
+linsolve = LinearSolve.set_A(sol2.cache,A2)
 sol3 = solve(linsolve)
 
 sol3.u
@@ -68,6 +68,10 @@ sol3.u
 =#
 ```
 
+The factorization occurs on the first solve, and it stores the factorization in
+the cache. You can retrieve this cache via `sol.cache`, which is the same object
+as the `init` but updated to know not to re-solve the factorization.
+
 The advantage of course with using LinearSolve.jl in this form is that it is
 efficient while being agnostic to the linear solver. One can easily swap in
 iterative solvers, sparse solvers, etc. and it will do all of the tricks like