https://docs.oracle.com/en/java/javase/12/tools/java.html
Section: -XX:FlightRecorderOptions

1)
"Note:
The -XX:+FlightRecorder option is no longer required to use JFR. This was a change made in JDK 8u40."
CORRECTION: AFAIK JVM dropped the requirement for -XX:+FlightRecorder when JFR was open sourced, which is JDK 11 (NOT in JDK8u40).

2)
java -XX:FlightRecorderOptions=allow_threadbuffers_to_disk=true -version
CORRECTION: Remove this parameter
Using this parameter results in the following error (JDK 12+33 2019-03-19):
    [0.079s][error][arguments] -XX:FlightRecorderOptions=disk=... has been removed. Use -XX:StartFlightRecording=disk=... instead.


3) threadbuffersize=size
    Please mention that the minimum size required is 4096

4) When Flight Recorder was open sourced with JDK 11 the syntax for the command line option -XX:StartFlightRecording was extended.
Previously the syntax was:

 -XX:StartFlightRecording=parameter=value.

From JDK 11, the following syntax is also allowed.

-XX:StartFlightRecording:parameter=value

similar to unified logging (-Xlog:). It may make sense to call out that there are two ways, as there may be users with older releases and/or scripts that want to use the previous syntax.