Add missing_docs lint and supply documents.

[jmjoy]

1. Add missing_docs lint in lib.rs.
2. Supply documents.
3. Fix some spell errors.

[codecov-commenter]

Codecov Report

Merging #49 (cd73b99) into master (7d344de) will not change coverage.
The diff coverage is 7.14%.

@@           Coverage Diff           @@
##           master      #49   +/-   ##
=======================================
  Coverage   21.40%   21.40%           
=======================================
  Files          20       20           
  Lines         612      612           
=======================================
  Hits          131      131           
  Misses        481      481           

Impacted Files	Coverage Δ
src/common/random_generator.rs	100.00% <ø> (ø)
src/common/system_time.rs	0.00% <ø> (ø)
src/logging/logger.rs	0.00% <0.00%> (ø)
src/logging/record.rs	0.00% <0.00%> (ø)
src/management/instance.rs	0.00% <ø> (ø)
src/metrics/meter.rs	0.00% <ø> (ø)
src/metrics/metricer.rs	0.00% <ø> (ø)
src/reporter/grpc.rs	0.00% <0.00%> (ø)
src/reporter/mod.rs	0.00% <ø> (ø)
src/reporter/print.rs	36.84% <ø> (ø)
... and 7 more

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