runtime: audit lock/atomic-free accesses in pollDesc

[prattmic]

#48925 identified a bug that was ultimately due to non-atomic access to pollDesc.rg/pollDesc.wg. This is fixed by https://golang.org/cl/355952, but pollDesc is full of other fields accessed non-atomically which may be problematic (everr stands out to me as suspicious), plus many fields that are almost always protected by the lock.

We should vet the current fields for issues, and clean up the documentation and semantics for better maintainability.

cc @mknyszek