The diagnostics dumped by the reader concurrency semaphore are pretty
common-sight in logs, as soon as a node becomes problematic. The reason
is that the reader concurrency semaphore acts as the canary in the coal
mine: it is the first that starts screaming when the node or workload is
unhealthy. This patch adds documentation of the content of the
diagnostics and how to diagnose common problems based on it.
Fixes: #10471Closes#11970
