We should add some text similar to the following to the manual, where troubleshooters will see it as a tool in the toolbox:

"On Linux (currently supported only for x86-64 architectures), we have introduced a runtime diagnostic feature in v4.3.4+. When a mongod or mongos process receives a SIGUSR2 signal, it will log a backtrace for each of its threads. Such log lines can be piped into buildscripts/mongosymb.py for expansion into more precise code locations."