Update force_unicode documentation
Should resolve #2053.

For consistency, I've re-added the `force_unicode` parameter to `to_latex()` and have it now raise a deprecation warning. I've also updated the common docstring for the `to_*` methods to have a brief explanation as to why the parameter is deprecated.

There were a couple other missing deprecation warnings (`force_unicode` in `to_html()` and `colSpace` in `to_latex()`) that should now have consistent behavior with other methods.
