Add comment on GMRES solver (#4254)

Additional comment on how the preconditioners in GMRES should be
implemented.

Src/LinearSolvers/AMReX_GMRES.H

@@ -65,8 +65,9 @@ namespace amrex {
  *               returns the 2-norm of v. For example, `return v.norm2()`.
  *
  *             - `void precond(V& lhs, V const& rhs)`\n
- *               applies preconditioner to rhs. If there is no preconditioner,
- *               this function should do lhs = rhs.
+ *               applies right-preconditioning, i.e.,
+ *               solve P(lhs) = rhs, where P is an approximation to A
+ *               If there is no preconditioner, P = I and thus this function should do lhs = rhs.
  *
  *             - `void scale(V& v, RT fac)`\n
  *               scales v by fac. For example, `v.mult(fac)`.