Extend split plugin documentation
Extend split plugin documentation

Summary:
Made dummy thread platform the default thread platform.
Gone over the documentation in a couple of iterations.

Temporarily closes T136.
T136 should be reopened after the 4.9.0 release because split does not yet
have an example.

Test Plan:

The split plugin still works after this change, and dummy is the default:
```
$ time mha srate=16000 iolib=MHAIOFile io.in=mha/Audiofiles/male_long.wav mhalib=mhachain "mha.algos=[route split]" "mha.route.out=[:0 :0]" "mha.split.algos=[cpuload:cpu0 cpuload:cpu1]" mha.split.cpu0.factor=10 mha.split.cpu1.factor=10 "mha.split.channels=[1 1]" io.out=/dev/null cmd=start cmd=quit
real	0m1.609s
user	0m1.595s
sys	0m0.012s
time mha srate=16000 iolib=MHAIOFile io.in=mha/Audiofiles/male_long.wav mhalib=mhachain "mha.algos=[route split]" "mha.route.out=[:0 :0]" "mha.split.algos=[cpuload:cpu0 cpuload:cpu1]" mha.split.cpu0.factor=10 mha.split.cpu1.factor=10 "mha.split.channels=[1 1]" io.out=/dev/null mha.split.thread_platform=posix cmd=start cmd=quit
real	0m0.965s
user	0m1.805s
sys	0m0.036s
```

Reviewers: pmaanen, hendrikk

Differential Revision: https://dev.openmha.org/D135

HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA