resolve issue #1374: document color spec.

[ben-schulz]

Fixes #1374

Contributor checklist:

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've squash'ed or fixup'ed junk commits with git-rebase
• I've built the docs and confirmed these changes don't cause new errors

Proposed changes

and

• add hyperlink to table in Colors.jl,
• note the Colorsjl library as the point-of-truth.
• incorporate revision suggested by @Mattriks.

[codecov-io]

Codecov Report

Merging #1388 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1388   +/-   ##
=======================================
  Coverage   90.27%   90.27%           
=======================================
  Files          39       39           
  Lines        4207     4207           
=======================================
  Hits         3798     3798           
  Misses        409      409

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update aec6dad...2c2072a. Read the comment docs.

[ben-schulz]

also addresses GiovineItalia/Compose.jl#380

[bjarthur]

thanks for this!

also, in general it is preferred to make a change to an existing PR by using git rebase and git push --force than to close the PR and open a new one. not that big of a deal, but it's easier to keep track of comments that way.

[ben-schulz]

my mistake; i'll follow that procedure next time. thanks!