[docs] Add activation checkpointing information

[SeanNaren]

What does this PR do?

Closes #9144
Closes #9152

Updates documentation to explain usage of activation checkpointing a bit better!

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[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #9165 (0873ce0) into master (e133061) will increase coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #9165    +/-   ##
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  Coverage      92%     92%            
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  Files         176     176            
  Lines       14804   15083   +279     
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+ Hits        13651   13929   +278     
- Misses       1153    1154     +1