Add chunkSize config to documentation

[ghost]

Fixes #4468

Ref: #4354 (comment) and https://github.com/owncloud/client/blob/v2.1.1/src/libsync/configfile.cpp#L127

[guruz]

@RealRancor wow, you're not listed as "contributor" yet? :) I thought you were.
I can not accept the request (sorry) if you don't send that contributor agreement to @karlitschek :(

[ghost]

@guruz Sorry, i try to avoid sending personal stuff around to companies. -> closed

[MorrisJobke]

I can not accept the request (sorry) if you don't send that contributor agreement to @karlitschek :(

MIT licensed is also fine. And as this is only a one line change we don't even need anything.

[ghost]

Ok, thought you have even stricter rules here at client. :-)

So if its needed: MIT licensed

[tflidd]

The OWNCLOUD_MAX_PARALLEL variable hasn't been documented either (if still valid):
#2493 (comment)

[ghost]

The chunksize was also available since longer time via an environment variable like the one above. Both were not documented since a longer time