Clarify documentation on lossless RGB

[BioTurboNick]

Fixes #364

[codecov]

Codecov Report

Merging #366 (dfd5622) into master (c3b6729) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #366   +/-   ##
=======================================
  Coverage   78.61%   78.61%           
=======================================
  Files          10       10           
  Lines        1253     1253           
=======================================
  Hits          985      985           
  Misses        268      268           

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