Add developer documentation for unlifted mutable arrays

[utdemir]

Unlifted mutable arrays, implemented in PR https://github.com/tweag/linear-base/pull/187, has to do some unsafe calls to runRW#; which has some nuances to using it right, as discussed on the PR comments. So, we should add some documentation explaining why we're doing the things we're doing there. It probably should be something along these lines: https://github.com/tweag/linear-base/pull/187#discussion_r489356315