Minor doc fixes

[akihironitta]

What does this PR do?

Includes minor fixes and improvements in the docs which I find while reading.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

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Codecov Report

Merging #301 (4729518) into master (f12f55c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #301   +/-   ##
=======================================
  Coverage   86.91%   86.91%           
=======================================
  Files          78       78           
  Lines        4021     4021           
=======================================
  Hits         3495     3495           
  Misses        526      526           

Flag	Coverage Δ
unittests	86.91% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/data/data_pipeline.py	92.43% <ø> (ø)
flash/core/data/process.py	94.20% <ø> (ø)
flash/core/model.py	87.04% <ø> (ø)
flash/core/trainer.py	79.36% <ø> (ø)

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