[DOC] fix #579

[szha]

Description

fix #579 by adding warning about trainer.step usage.

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

• add doc fix

[mli]

Job PR-678/8 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-678/8/index.html

[codecov]

Codecov Report

Merging #678 into master will increase coverage by 0.98%.
The diff coverage is 7.47%.

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
+ Coverage   63.83%   64.81%   +0.98%     
==========================================
  Files         143      144       +1     
  Lines       13467    13253     -214     
==========================================
- Hits         8596     8590       -6     
+ Misses       4871     4663     -208

Flag	Coverage Δ
#PR665	?
#PR669	65.68% <33.33%> (?)
#PR672	64.71% <7.47%> (?)
#PR678	65.06% <7.47%> (?)
#master	65.01% <7.47%> (+1.46%)	⬆️
#notserial	41.57% <4.12%> (+0.29%)	⬆️
#py2	64.53% <7.47%> (+0.94%)	⬆️
#py3	64.62% <7.47%> (+0.91%)	⬆️
#serial	50.94% <5.41%> (+1.03%)	⬆️

[codecov]

Codecov Report

Merging #678 into master will increase coverage by 0.98%.
The diff coverage is 7.47%.

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
+ Coverage   63.83%   64.81%   +0.98%     
==========================================
  Files         143      144       +1     
  Lines       13467    13253     -214     
==========================================
- Hits         8596     8590       -6     
+ Misses       4871     4663     -208

Flag	Coverage Δ
#PR665	?
#PR669	65.68% <33.33%> (?)
#PR672	64.71% <7.47%> (?)
#PR678	65.06% <7.47%> (?)
#master	65.01% <7.47%> (+1.46%)	⬆️
#notserial	41.57% <4.12%> (+0.29%)	⬆️
#py2	64.53% <7.47%> (+0.94%)	⬆️
#py3	64.62% <7.47%> (+0.91%)	⬆️
#serial	50.94% <5.41%> (+1.03%)	⬆️