[doc] Update documentation on the topic of filtering tests by system

[rsarm]

Closes #490

[vkarak]

The documentation does not state how one can show the tests of a different system or of all the systems. It implies that there is no control over the listing per system. I will fine tune it.

[vkarak]

@jenkins-cscs retry daint

[codecov-io]

Codecov Report

Merging #492 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
- Coverage   91.42%   91.39%   -0.03%     
==========================================
  Files          70       70              
  Lines        8788     8788              
==========================================
- Hits         8034     8032       -2     
- Misses        754      756       +2

Impacted Files	Coverage Δ
reframe/core/config.py	82.45% <0%> (-1.76%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 49f03d6...98cddec. Read the comment docs.