I have installed mono version 18.104.22.168 on my CentOS operating system from this source by first downloading the tarball and then doing
sudo make install
GC: sgen and Included Boehm GC with typed GC and parallel mark
Engine: Building and using the JIT
LLVM Back End: no (dynamically loaded: no)
Warning: In environment variable 'MONO_GC_PARAMS': Unknown major collector 'marksweep-par'. - Using 'marksweep' instead.
You are referring to a old man page (Mono 2.5) while you are running 4.2+.
You should review the man page installed on your system:
Specifies which major collector to use. Options are
marksweepfor the Mark&Sweep collector, and
marksweep-concfor concurrent Mark&Sweep. The non-concurrent Mark&Sweep collector is the default.