Information on incremental checkpoints is somewhat scattered and hard to find in the documentation. In particular, the main page on checkpoints  should contain at least a short description of incremental checkpoints. They are currently mentioned in only one sentence, without any further explanation.
A very short summary with a link to the RocksDB section  would suffice, since they are specific to that state backend.
They are also mentioned on the config pages [3 and 4], which likewise contain incomplete information and no link ("may not be supported by every state backend").
One basically has to follow the above clues and deduce that there may be more information in the state backends section.