[bugfix] Fix documentation of sanity functions accepting a filename

[vkarak]

Fixes #1646.

[codecov-io]

Codecov Report

Merging #1666 (f849866) into master (01d7efc) will decrease coverage by 0.29%.
The diff coverage is 45.28%.

@@            Coverage Diff             @@
##           master    #1666      +/-   ##
==========================================
- Coverage   87.54%   87.24%   -0.30%     
==========================================
  Files          45       45              
  Lines        7492     7545      +53     
==========================================
+ Hits         6559     6583      +24     
- Misses        933      962      +29     

Impacted Files	Coverage Δ
reframe/utility/sanity.py	97.45% <ø> (ø)
reframe/core/modules.py	56.62% <45.28%> (-1.21%)	⬇️

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