cp-sat: fix typos and improve docstrings in cp_model.py

[chripiermarini]

Hello, I wanted to started contributing to or-tools as I use it daily and I would like to help :)

1. Fixed broken doc link #lineacp_model.LinearExpr → #cp_model.LinearExpr
2. Removed duplicate UNKNOWN assignment
3. Fixed typo "determistic" → "deterministic" in CpSolverSolutionCallback.deterministic_time docstring
4. Fixed float_value() docstring saying "An integer value" → "A float value" (copy-paste error)
5. Added missing input validation to add_reservoir_constraint — its docstring promised ValueError raises for invalid min_level/max_level/empty times, but the code had none (unlike its sibling add_reservoir_constraint_with_active which validates properly)
6. Fixed grammar "an linear" → "a linear" in CpSolverSolutionCallback.value() docstring
7. Added missing docstrings to has_objective() and clear_objective()