Document when WDL workflows copy or mount files

[unito-bot]

WDL users need to know whether their workflow containers are seeing workflow input files mounted from their original locations, or copies local to their node, or copies made specifically for each task, in order to write workflows that are efficient.

We should document what happens on the Phoenix cluster with the default settings, and what settings can be used to change this (like turning caching on or off) to work around problems.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1563