Document use of LAUNCHER_DEBUG variable as an alternative to --debug-launcher option

The --debug-launcher nuxeoctl option purpose is to "Activate Java debugging mode on the Launcher". That option sets the following environment variable:
LAUNCHER_DEBUG="-Xdebug -Xrunjdwp:transport=dt_socket,address=8788,server=y,suspend=y"

As an alternative, it is possible to manually set the LAUNCHER_DEBUG variable with custom parameters.
That feature is used, for instance, to work around NXP-20189:
LAUNCHER_DEBUG="-Djvmcheck=nofail" nuxeoctl start

Accordingly update nuxeoctl help.
Note that the --debug-launcher option is not available on Windows while the LAUNCHER_DEBUG usage is possible.