[DOC] add TensorflowSavedModelArtifact doc

[parano]

• add/improve TensorflowSavedModelArtifact doc
• update artifact doc string

[codecov-io]

Codecov Report

Merging #368 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #368   +/-   ##
=======================================
  Coverage   53.55%   53.55%           
=======================================
  Files          80       80           
  Lines        5368     5368           
=======================================
  Hits         2875     2875           
  Misses       2493     2493

Impacted Files	Coverage Δ
bentoml/artifact/sklearn_model_artifact.py	37.5% <ø> (ø)	⬆️
bentoml/artifact/pickle_artifact.py	96.77% <ø> (ø)	⬆️
bentoml/artifact/tf_savedmodel_artifact.py	37.7% <ø> (ø)	⬆️
bentoml/artifact/pytorch_model_artifact.py	40.54% <ø> (ø)	⬆️
bentoml/artifact/fastai_model_artifact.py	43.58% <0%> (ø)	⬆️
bentoml/artifact/xgboost_model_artifact.py	40% <0%> (ø)	⬆️
bentoml/artifact/keras_model_artifact.py	22.4% <0%> (ø)	⬆️

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