Summary:
The patch makes the following changes to the API comments:
* Some general comments about snapshots, thread safety, and user-defined timestamps are moved to a more prominent place at the top of the file.
* Detailed descriptions are added for each `ValueType` and `Decision`, fixing and extending some existing comments (e.g. that of `kRemove`, which suggested that key-values are simply removed from the output, while in reality base values are converted to tombstones) and adding detailed comments that were missing (e.g. `kPurge` and `kChangeWideColumnEntity`).
* Updated/extended the comments of `FilterV2/V3` and `FilterBlobByKey`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11261
Reviewed By: akankshamahajan15
Differential Revision: D43714314
Pulled By: ltamasi
fbshipit-source-id: 835f4b1bdac1ce0e291155186095211303260729