Document XDG_* options for --new-instance

Document XDG_* options for --new-instance

Update the man page and ref. document to include:

When the --remote-instance option is used, a single set of configuration
files is used for all instances.
However, the environment variables XDG_CONFIG_HOME, XDG_CACHE_HOME,
XDG_DATA_HOME can be used to modify this behavior on an individual basis
e.g.

XDG_CONFIG_HOME=/tmp/a XDG_CACHE_HOME=/tmp/b geeqie