Add docs for values and indices retrieval from IdOffsetRange

[mkitti]

This is a reduced version of #316 by simply documenting how to retrieve the values and indices shown by Base.show.

Added Documentation

The values and indices keywords can be extracted via Base.values or Base.eachindex and converted to a UnitRange.

julia> values(ro)
IdOffsetRange(values=9:11, indices=-1:1)
julia> eachindex(ro)
IdOffsetRange(values=-1:1, indices=-1:1)
julia> UnitRange(values(ro))
9:11
julia> convert(UnitRange, eachindex(ro))
-1:1

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.67%. Comparing base (08dc371) to head (341f8cc).
Report is 37 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #317   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files           5        5           
  Lines         452      452           
=======================================
  Hits          446      446           
  Misses          6        6           

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