Added missing package document and canonical import paths

[rakyll]

Also simplified the first lines of some package docs
to focus on what the package provides instead of
explaining the implementation details.

[codecov]

Codecov Report

Merging #1125 into master will increase coverage by 6.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1125     +/-   ##
========================================
+ Coverage    71.2%   77.2%   +6.0%     
========================================
  Files          68     133     +65     
  Lines        3709    7101   +3392     
========================================
+ Hits         2643    5487   +2844     
- Misses        945    1365    +420     
- Partials      121     249    +128     

Impacted Files	Coverage Δ
api/trace/tracetest/tracer.go	100.0% <ø> (ø)
label/type_string.go	12.5% <0.0%> (-37.5%)	⬇️
api/metric/kind_string.go	16.6% <0.0%> (-33.4%)	⬇️
api/metric/numberkind_string.go	25.0% <0.0%> (-25.0%)	⬇️
exporters/stdout/config.go	72.0% <0.0%> (ø)
exporters/otlp/options.go	51.7% <0.0%> (ø)
sdk/internal/codes.go	100.0% <0.0%> (ø)
sdk/export/trace/tracetest/test.go	100.0% <0.0%> (ø)
sdk/metric/aggregator/lastvalue/lastvalue.go	83.7% <0.0%> (ø)
sdk/metric/controller/pull/config.go	50.0% <0.0%> (ø)
... and 59 more

[MrAlias]

🎉