Doc: Adds documentation for the dataset compression argument from #1341 and #1250

[eddiebergman]

This adds some documentation to the manual and FAQ about how to use dataset_compression as well as update an example which used predefined splits. Here, uses should now about dataset_compression as this will invalidate predefined splits.

Also does some slight tidy of the arguments w.r.t. to how it shows in the online documentation. In general, it seems to many bold parts interfere with parameter definitions which also appear bold and at the same indent.

Also, recomments back in the "Manual" tab which was meant to be fixed some time prior as far as I recall.

[codecov]

Codecov Report

Merging #1386 (03ac909) into development (bdebeca) will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #1386   +/-   ##
============================================
  Coverage        88.42%   88.42%           
============================================
  Files              140      140           
  Lines            11090    11090           
============================================
  Hits              9806     9806           
  Misses            1284     1284           

Impacted Files	Coverage Δ
autosklearn/estimators.py	93.80% <ø> (ø)

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