Rewritten the doc for more clarity and explicit mention of GUI tools. The R4.0 section was needlessly complicated and seemed more scary than the procedure really is (resize and don't worry, in most cases).
Depends on https://github.com/QubesOS/qubes-attachment/pull/17
adjust title to "Private" vs. counterpart document "Root"
add 4.0 content
add placeholder procedure for Shrinking private disk image (Linux VM, R4.0)
remove outdated, root.img specific content
misc. spelling/grammar
This page has a bit of overlap with the Resize Root Disk Image page; the latter is a bit more clear and specific, and also describes caveats such as to avoid changes exceeding 10G between reboots. Added link to the other page, and updated some of the guidance to be more clear.
