Minor documentation fix

Add automatic generated fortran support for setting and returning contexts

Major cleanup of the code for setting and returning application (previously called user) contexts
- PetscCtx ctx is now t

Add automatic generated fortran support for setting and returning contexts

Major cleanup of the code for setting and returning application (previously called user) contexts
- PetscCtx ctx is now the standard for passing in contexts
- PetscCtxRt ctx is the standard for passing out contexts, including context destroy routines based on PetscCtxDestroyFn

Both of these are typedef to void*

Also removed most use of user and userctx for context arguments now consistently named ctx

Improved documentation on PetscCtx and PetscCtxRt and on how to use contexts (including getting them) from Fortran

show more ...

Fix terminology for Pmat in KSPView output.

Make clearer the relationship between KSPSetOperators and PCSetOperators.

Remove left over parts of PCSetPreSolve() that were mistakenly left in !8419

Change use of void (*)(void) and void (**)(void) in the PETSc API to PetscVoidFn * and PetscVoidFn ** or PetscErrorCodeFn * or PetscErrorCodeFn **

Merge branch 'adams/fix-mg-reusepc' into 'main'

set fine grid smoother 'reusepc' with mgpc->reusepc

See merge request petsc/petsc!8656