Package man page cleanups

[mbojan]

In summary

• I removed a section "Introduction". The same paragraph is now in a automatically generated section "Description".
• Grouped Roxygen tags in a more meaningful way.
• Changed section title from "oai package details" to "Implementation details".

[codecov-io]

Codecov Report

Merging #50 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #50   +/-   ##
=======================================
  Coverage   67.38%   67.38%           
=======================================
  Files          16       16           
  Lines         420      420           
=======================================
  Hits          283      283           
  Misses        137      137

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[sckott]

looks good!