Why document?
I've gotten that advice, but I've almost never seen it to be helpful:
- It's almost impossible to win a law suit for being fired, unless it's clearly based on sex/race/etc.
- If you sue, you'll burn bridges
- If future employers find out you sued, they won't want to hire you.
It's one of those things that's nice in abstract, but in practice, seems like a waste of time, relative to practising for interviews, applying for jobs, or trying to fix things with the old employer.
My advice is to jump ship, and leave on good terms.
Companies aren't people. I once worked for a company which wasn't a great employer, but I'm glad I left on good terms. The place no longer exists, and there are lots of people there whom I value.