Add documentation for JFR options

[tajila]

Issue or pull request number:
eclipse-openj9/openj9#20854

Overview:
JFR is a tool built-in to the JVM that collects profiling and diagnostic information. If JFR is enabled on the JVM (via -XX:[+|-]FlightRecorder) then a user can trigger profile and diagnostic recording with jcmd (https://eclipse.dev/openj9/docs/tool_jcmd/). Note, recording does not start automatically, it must be triggered.

To start a recording one can do the following:
jcmd [JDK ID] JFR.start [filename=...] [duration=..s]

The optional filename=.. can be used to specify the name of the file where the recording will be saved to.
The optional duration=.. can be used to specify the duration of the recording. Units are seconds, minutes, hours and days (s, m, h and d respectively).

Note: in 0.49 OpenJ9 provides limted support for JFR. This means that a subset of the JFR events ar supported and a subset of the JFR jcmd options are supported.

Release target:
Eclipse OpenJ9 0.49.0

Applies to the following JDK versions:
11 and later

Applies to the following platforms:
Linux X|A

For new command line options:

• Option name and syntax: -XX:[+|-]FlightRecorder
• Purpose: Toggles if JFR is enable on the JVM. By default it is enabled.
• Restrictions: None