Update outdated docstrings

[renatomello]

A lot of the older docstrings do not match the new functionalities added on qibo in the last $2$ or so years. This PR updates them.

One of the main goals is to discourage people who first interact with the docs from getting in the habit of using unnecessarily short variable names, which leads to issues when the same people then start contributing code in PRs.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.72%. Comparing base (a8e7edd) to head (3110f99).
Report is 31 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1512      +/-   ##
==========================================
- Coverage   99.72%   99.72%   -0.01%     
==========================================
  Files          80       80              
  Lines       11784    11783       -1     
==========================================
- Hits        11752    11751       -1     
  Misses         32       32              

Flag	Coverage Δ
unittests	99.72% <100.00%> (-0.01%)	⬇️

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[MatteoRobbiati]

Thanks! Some minor suggestions follow.

[MatteoRobbiati]

Thanks!