Merge pull request #15128 from bangerth/77-2

In step-77, also talk about the other nonlinear solver packages we support.

examples/step-77/doc/intro.dox

@@ -155,6 +155,18 @@ provides a vast amount of finite-element functionality, %SUNDIALS'
 KINSOL package provides a vast amount of nonlinear solver
 functionality, and we better use it.
 
+@note While this program uses SUNDIAL's KINSOL package as the engine to
+  solve nonlinear problems, KINSOL is not the only option you have.
+  deal.II also has interfaces to PETSc's SNES collection of algorithms
+  (see the PETScWrappers::NonlinearSolver class) as well as to
+  the Trilinos NOX package (see the TrilinosWrappers::NOXSolver class)
+  that provide not only very similar functionality, but also a largely
+  identical interface. If you have installed a version of deal.II that
+  is configured to use either PETSc or Trilinos, but not SUNDIALS,
+  then it is not too difficult to switch this program to use either
+  of the former two packages instead: Basically everything that we
+  say and do below will also be true and work for these other packages!
+
 
 <h3> How deal.II interfaces with KINSOL </h3>
 
@@ -237,7 +249,7 @@ problem as follows:
   independent, and can be broken into separate functions.
 
 There is an assumption here that whenever KINSOL asks for a linear
-solver with the (approximation of the) Jacobian, that this will be for
+solver with the (approximation of the) Jacobian, that this will be
 for an update $\delta U$ (which has zero boundary values), a multiple
 of which will be added to the solution (which already has the right
 boundary values).  This may not be true and if so, we might have to