I submitted an initial review / comments in the PR. I think this is good for a first iteration in order to understand what APIs to expose publicly (also, keep in mind I may not have a full picture of how this is intended to be used precisely).

As for the doc, I find it a bit too verbose. If the namespace change is accepted consider doing the following:

* document `SharedMemoryManager` right after `SyncManager`. That would let you reuse the description for `Semaphore`, `Barrier`, `Lock`, etc.
* within `SharedMemoryManager` doc link/point to a different section of the doc where you explain the whole thing in more details, and also document the remaining APIs 

Just an idea. Writing good doc is not easy and it requires actually putting hands on it. 

Hope this helps.