otlp: Refine documentation

[pellared]

Fixes #4688

Testing:

go install golang.org/x/pkgsite/cmd/pkgsite@latest
pkgsite

[codecov]

Codecov Report

Merging #4695 (07e2a4d) into main (0f5179f) will decrease coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main   #4695     +/-   ##
=======================================
- Coverage   81.8%   81.8%   -0.1%     
=======================================
  Files        225     225             
  Lines      18087   18087             
=======================================
- Hits       14798   14796      -2     
- Misses      2992    2994      +2     
  Partials     297     297             

Files	Coverage Δ
exporters/otlp/otlpmetric/otlpmetricgrpc/config.go	41.8% <ø> (ø)
exporters/otlp/otlptrace/otlptracegrpc/options.go	76.4% <ø> (ø)

... and 1 file with indirect coverage changes

[dashpole]

I like the example cleanups a lot.