Add warning for missing documentation

[elegaanz]

To force us to document our code.

I didn't added it to binaries (src/ and plume-cli/ because it didn't really made sense for them IMO), but I can.

Also, to keep this PR small, I propose that we slowly document each function as we modify them in other PRs. Does it seem reasonable to you?

[igalic]

can't we add that that somewhere to clippy or check?

[elegaanz]

Isn't there a better way to allow just these warnings in clippy?

[codecov[bot]]

Codecov Report

Merging #727 into master will decrease coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #727      +/-   ##
==========================================
- Coverage   39.17%   39.15%   -0.03%     
==========================================
  Files          73       73              
  Lines        9653     9653              
  Branches     2183     2182       -1     
==========================================
- Hits         3782     3780       -2     
  Misses       4819     4819              
- Partials     1052     1054       +2