Fix LinearVariationalSolver docstring

solver_parameters, not parameters.
Add backwards-compatibility for parameters keyword to FooSolver

Accept both parameters and solver_parameters, preferring the latter and
warning if the former is used.
Merge pull request #345 from firedrakeproject/parameters-compat

* parameters-compat:
  Add backwards-compatibility for parameters keyword to FooSolver
  Fix LinearVariationalSolver docstring