Docs: remove non-existing API

This requires some changes to user FORTRAN code

Pass PETSC_NULL_XXX_ARRAY when passing a NULL as an argument that is expecting an array
Pass PETSC_NULL_ENUM when argument returns an enum instead of

This requires some changes to user FORTRAN code

Pass PETSC_NULL_XXX_ARRAY when passing a NULL as an argument that is expecting an array
Pass PETSC_NULL_ENUM when argument returns an enum instead of PETSC_NULL_INTEGER
Pass arrays (and not scalar values) when the argument is expecting an array; this means replace, for example, the argument v with [v]
Use PetscObjectIsNull(obj) to check if the object is NULL, instead of obj == PETSC_NULL_XXX

The compiler will now automatically prevent you from using the wrong argument type for the first three bullets above
This will also require an update sowing with the new sowing branch this MR is using.

show more ...

Merge remote-tracking branch 'origin/release'

Docs: fix double backticks in seealso