Update Trainer flag docs for strategy

[kaushikb11]

What does this PR do?

Follow up to #10000

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[tchaton]

Should we indicate training type plugin will be renamed Strategy for v1.6 ?

[codecov]

Codecov Report

Merging #10042 (6e8c162) into master (f0e6f1b) will decrease coverage by 0%.
The diff coverage is n/a.

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##           master   #10042   +/-   ##
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- Coverage      93%      93%   -0%     
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  Files         180      180           
  Lines       15900    15892    -8     
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- Hits        14731    14722    -9     
- Misses       1169     1170    +1