Adds docstrings to buttons

[MarcSkovMadsen]

This PR adds missing docstrings to buttons.

This makes it so much easier to use the buttons in an IDE like VS Code.

[codecov]

Codecov Report

Merging #3352 (43dbc38) into master (4d4afb7) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 43dbc38 differs from pull request most recent head fae8839. Consider uploading reports for the commit fae8839 to get more accurate results

@@           Coverage Diff           @@
##           master    #3352   +/-   ##
=======================================
  Coverage   83.27%   83.27%           
=======================================
  Files         196      196           
  Lines       26607    26607           
=======================================
  Hits        22156    22156           
  Misses       4451     4451           

Impacted Files	Coverage Δ
panel/widgets/button.py	94.20% <100.00%> (ø)

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