add the docstrings to all tuners

[haifeng-jin]

Resolves #704

[codecov-commenter]

Codecov Report

Base: 95.11% // Head: 95.11% // No change to project coverage 👍

Coverage data is based on head (88793dd) compared to base (758b5ce).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #769   +/-   ##
=======================================
  Coverage   95.11%   95.11%           
=======================================
  Files          46       46           
  Lines        3088     3088           
=======================================
  Hits         2937     2937           
  Misses        151      151           

Impacted Files	Coverage Δ
keras_tuner/tuners/bayesian.py	94.59% <ø> (ø)
keras_tuner/tuners/gridsearch.py	100.00% <ø> (ø)
keras_tuner/tuners/hyperband.py	96.29% <ø> (ø)
keras_tuner/tuners/randomsearch.py	94.11% <ø> (ø)

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