Improve Documentation

[tri-adam]

Improve siftool documentation by adding Long descriptions (used unconditionally by singularity), and improving consistency of terminology across commands.

[codecov-commenter]

Codecov Report

Merging #43 (cc04e26) into master (66d2e60) will increase coverage by 0.57%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   66.50%   67.08%   +0.57%     
==========================================
  Files          25       25              
  Lines        1651     1680      +29     
==========================================
+ Hits         1098     1127      +29     
+ Misses        362      361       -1     
- Partials      191      192       +1     

Impacted Files	Coverage Δ
pkg/siftool/add.go	62.83% <100.00%> (+3.60%)	⬆️
pkg/siftool/del.go	83.33% <100.00%> (+3.33%)	⬆️
pkg/siftool/dump.go	83.33% <100.00%> (+3.33%)	⬆️
pkg/siftool/header.go	100.00% <100.00%> (ø)
pkg/siftool/info.go	83.33% <100.00%> (+3.33%)	⬆️
pkg/siftool/list.go	100.00% <100.00%> (ø)
pkg/siftool/new.go	100.00% <100.00%> (ø)
pkg/siftool/setprim.go	83.33% <100.00%> (+3.33%)	⬆️
pkg/siftool/siftool.go	91.66% <100.00%> (+2.19%)	⬆️

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[dtrudg]

LGTM