[DOC] Update documentation for SplitSampler

[eric-haibin-lin]

Description

As title

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[codecov]

Codecov Report

Merging #603 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #603   +/-   ##
=======================================
  Coverage   71.12%   71.12%           
=======================================
  Files         126      126           
  Lines       10994    10994           
=======================================
  Hits         7820     7820           
  Misses       3174     3174

Flag	Coverage Δ
#PR594	71.21% <0%> (ø)	⬆️
#PR600	71.21% <0%> (ø)	⬆️
#PR603	71.21% <0%> (ø)	⬆️
#master	71.21% <0%> (ø)	⬆️
#notserial	49.03% <0%> (ø)	⬆️
#py2	70.91% <0%> (ø)	⬆️
#py3	70.94% <0%> (ø)	⬆️
#serial	54.63% <0%> (ø)	⬆️

[mli]

Job PR-603/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-603/2/index.html

[mli]

Job PR-603/3 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-603/3/index.html