froll manual improvements for vectorized input

[jangorecki]

I intentionally removed "vector" to keep description of the arguments simpler, and then at its end, when non-vectorized input is more easily described, I mention about possibility to vectorize.
I believe it is not completely ideal but at least improvement towards clarity.

closes #7296

diff: frolladapt2025...froll-nman

This work was supported by the Medical Research Council, UK [grant number 1573].

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (9b421c8) to head (2e9ae4e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7297   +/-   ##
=======================================
  Coverage   99.08%   99.08%           
=======================================
  Files          83       83           
  Lines       15780    15780           
=======================================
  Hits        15635    15635           
  Misses        145      145           

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[github-actions]

• HEAD=froll-nman stopped early for DT[by,verbose=TRUE] improved in #6296
• HEAD=froll-nman stopped early for DT[,.SD] improved in #4501
  

Generated via commit cff2b6d

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	4 minutes and 52 seconds
Installing different package versions	10 minutes and 29 seconds
Running and plotting the test cases	2 minutes and 48 seconds