add note on transparent - opaque conversions

[t-bltg]

transparent and opaque should be appear in doc search results.

[codecov[bot]]

Codecov Report

Merging #522 (445ed5f) into master (b550b7b) will decrease coverage by 0.23%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #522      +/-   ##
==========================================
- Coverage   94.33%   94.09%   -0.24%     
==========================================
  Files           9        9              
  Lines        1270     1270              
==========================================
- Hits         1198     1195       -3     
- Misses         72       75       +3     

Impacted Files	Coverage Δ
src/utilities.jl	95.60% <0.00%> (-0.74%)	:arrow_down:
src/conversions.jl	99.13% <0.00%> (-0.29%)	:arrow_down:

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[kimikage]

Ugh, we can't use color() in the context.