Add docs for evaluation

[yeukfu]

Close #2115.

[codecov]

Codecov Report

Merging #2151 (98918d6) into master (f856f74) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2151      +/-   ##
==========================================
+ Coverage   91.36%   91.46%   +0.09%     
==========================================
  Files         198      197       -1     
  Lines       10944    10845      -99     
  Branches     1374     1356      -18     
==========================================
- Hits         9999     9919      -80     
+ Misses        688      671      -17     
+ Partials      257      255       -2     

Impacted Files	Coverage Δ
src/garage/tf/_functions.py	55.12% <0.00%> (-13.73%)	⬇️
src/garage/np/algos/cma_es.py	96.92% <0.00%> (-3.08%)	⬇️
src/garage/sampler/multiprocessing_sampler.py	88.31% <0.00%> (-2.60%)	⬇️
src/garage/np/algos/cem.py	97.43% <0.00%> (-2.57%)	⬇️
...rage/tf/optimizers/conjugate_gradient_optimizer.py	81.90% <0.00%> (-1.81%)	⬇️
.../garage/torch/policies/deterministic_mlp_policy.py	91.17% <0.00%> (-1.69%)	⬇️
src/garage/experiment/experiment.py	91.84% <0.00%> (-1.01%)	⬇️
src/garage/tf/baselines/continuous_mlp_baseline.py	92.18% <0.00%> (-0.57%)	⬇️
src/garage/torch/policies/stochastic_policy.py	97.05% <0.00%> (-0.57%)	⬇️
src/garage/np/policies/fixed_policy.py	93.54% <0.00%> (-0.40%)	⬇️
... and 36 more

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[yeukfu]

@Mergifyio rebase

[mergify]

Command rebase: success

Branch has been successfully rebased