Added missing dartdoc to classes

[awhitford]

• Added missing dartdoc to classes.
• For TextLiquidFill, renamed WavePainter to be _WavePainter since the class is internal only -- and then missing docs are not critical.
• Related to [docs] Improve documentation and add dartdoc #147

[codecov]

Codecov Report

Merging #148 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #148   +/-   ##
=======================================
  Coverage   81.99%   81.99%           
=======================================
  Files           8        8           
  Lines         761      761           
=======================================
  Hits          624      624           
  Misses        137      137           

Impacted Files	Coverage Δ
lib/src/colorize.dart	75.90% <ø> (ø)
lib/src/fade.dart	76.40% <ø> (ø)
lib/src/rotate.dart	81.00% <ø> (ø)
lib/src/scale.dart	77.55% <ø> (ø)
lib/src/typer.dart	82.14% <ø> (ø)
lib/src/typewriter.dart	79.43% <ø> (ø)
lib/src/wavy.dart	92.43% <ø> (ø)
lib/src/text_liquid_fill.dart	88.88% <100.00%> (ø)

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[aagarwal1012]

Thanks for this quick PR @awhitford, it looks good to me. 👍