[Docs] update docs for resume_from_checkpoint

[rohitgr7]

What does this PR do?

if resume_from_checkpoint is done using a mid-epoch checkpoint, it resumes from the next epoch. Just added this info here.

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[codecov]

Codecov Report

Merging #5164 (a8799cf) into master (15a400b) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #5164   +/-   ##
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  Coverage      93%     93%           
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  Files         134     134           
  Lines        9950    9946    -4     
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- Hits         9261    9260    -1     
+ Misses        689     686    -3