Extra docs for wide format plotting

[Mattriks]

• I've updated the documentation to reflect these changes
• I've built the docs and confirmed these changes don't cause new errors

This PR

• closes StackOverflowError: Gadfly.draw() fails for large images #1342
• Adds extra documentation for wide-format plotting

[codecov-io]

Codecov Report

Merging #1390 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1390   +/-   ##
=======================================
  Coverage   90.27%   90.27%           
=======================================
  Files          39       39           
  Lines        4207     4207           
=======================================
  Hits         3798     3798           
  Misses        409      409

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