Add docs to *_processor modules

[whatyouhide]

This is work that I quickly chatter about with @tsloughter.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: whatyouhide / name: Andrea Leopardi (d5fe7e0)
• ✅ login: tsloughter / name: Tristan Sloughter (ce08c3c)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (05a7704) 74.35% compared to head (d5fe7e0) 74.35%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #675   +/-   ##
=======================================
  Coverage   74.35%   74.35%           
=======================================
  Files          56       56           
  Lines        1852     1852           
=======================================
  Hits         1377     1377           
  Misses        475      475           

Flag	Coverage Δ
api	73.14% <ø> (ø)
erlang	74.35% <ø> (ø)
exporter	67.47% <ø> (ø)
sdk	78.69% <ø> (ø)
zipkin	54.16% <ø> (ø)

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[ferd]

Docs changes are clearly unrelated to the CI failures, these are decent improvements.