[doc] Improve documentation of built-ins

[jjotero]

Closes #2067. This issue proposed that all hooks should be executed in the test's stagedir, but this cannot be generalised to all the pipeline stages (this could never apply to the init, setup and cleanup stages). Thus, instead it has been decided that this information is added in the docs, alongside a pseudo-example on how to change the working directory from the hook itself.

This PR also includes other minor improvements across the whole builtins section.

[vkarak]

Lgtm just a minor comment.

[codecov-commenter]

Codecov Report

Merging #2128 (a589dc7) into master (b5cae2d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2128   +/-   ##
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  Coverage   85.97%   85.97%           
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  Files          53       53           
  Lines        9362     9362           
=======================================
  Hits         8049     8049           
  Misses       1313     1313           

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