Expand godoc for remote.Tag

[jonjohnsonjr]

Fixes #862

Also, refactor unpackTaggable a bit to ensure we only call RawManifest once.

[codecov-io]

Codecov Report

Merging #863 (e6f96a3) into master (f39a196) will increase coverage by 0.00%.
The diff coverage is 57.14%.

@@           Coverage Diff           @@
##           master     #863   +/-   ##
=======================================
  Coverage   74.29%   74.29%           
=======================================
  Files         106      106           
  Lines        4454     4451    -3     
=======================================
- Hits         3309     3307    -2     
  Misses        651      651           
+ Partials      494      493    -1     

Impacted Files	Coverage Δ
pkg/v1/remote/write.go	65.59% <57.14%> (+0.34%)	⬆️
pkg/v1/remote/descriptor.go	73.78% <0.00%> (-0.61%)	⬇️

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