Safety in parallel builds

[hainest]

dcb06b0 marked rli safe for parallel reading, but not for parallel writing. I couldn't find any comments or documentation describing why that was chosen. I'm definitely not a sphinx expert, but a cursory look over the code it's not immediately clear to me where thread unsafe writing would occur. Anecdotally, enabling parallel writes in my build with a few hundred files and a couple dozen rli uses, I didn't see any problems.