update training type plugin docs regarding result caching

[awaelchli]

What does this PR do?

Follow up to #7232, some doc improvements are necessary.

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[codecov]

Codecov Report

Merging #7261 (f4bbc9d) into master (7a48db5) will increase coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #7261    +/-   ##
=======================================
  Coverage      91%     92%            
=======================================
  Files         199     199            
  Lines       12799   13696   +897     
=======================================
+ Hits        11701   12576   +875     
- Misses       1098    1120    +22     

[tchaton]

LGTM !