Summary:
**Context/Summary:**
The logic related to PositionedRead in SequentialFileReader::Read confused me a bit as discussed here https://github.com/facebook/rocksdb/pull/9973#discussion_r872869256. Therefore I added a drawing with help from cbi42.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/10002
Test Plan: - no code change
Reviewed By: anand1976, cbi42
Differential Revision: D36422632
Pulled By: hx235
fbshipit-source-id: 9a8311d2365564f90d216c430f542fc11b2d9cde