PR #1287 changed the default cache location on darwin and windows.
Update the changelog and manual to reflect the new behavior.
Since the cache hasn't been included in an official release yet, I've
just changed the main cache changelog entry.
Fixes #1309
This commit refactors the documentation according to my proposal in #1273
and the discussion I had with fd0 on IRC.
The bits from the manual that I could not immediately put into the new
structure are contained in manual_rest.rst Anything else is still there,
nothing has been deleted.
I changed the heading markup to follow the convention used in Python’s
Style Guide for documentation, this convention is explained in a comment
at the top of every file.
I also added a paragraph on installing restic on Debian.
