[bugfix] Register performance functions even if perf_variables are defined in the class body and improve the performance function docs

[vkarak]

Although this PR is primarily about the updating and improving the docs, I use the bug fix as a title, so that it shows up in the release notes.

Closes #2704.

[codecov-commenter]

Codecov Report

Base: 86.61% // Head: 86.57% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (f4a88b7) compared to base (def32e3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2770      +/-   ##
==========================================
- Coverage   86.61%   86.57%   -0.04%     
==========================================
  Files          60       60              
  Lines       11214    11226      +12     
==========================================
+ Hits         9713     9719       +6     
- Misses       1501     1507       +6     

Impacted Files	Coverage Δ
reframe/core/pipeline.py	93.76% <100.00%> (-0.01%)	⬇️
reframe/frontend/cli.py	73.22% <0.00%> (-0.45%)	⬇️
reframe/frontend/loader.py	96.02% <0.00%> (ø)
reframe/core/logging.py	81.02% <0.00%> (+0.03%)	⬆️

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