If the XDG_RUNTIME_DIR environment variable is set, we are being told by
the OS that this directory exists and is appropriately configured
already.  In the fallback case of ~/.cache/, however, the directory may
not yet exist.

Rework the logic of this function a little so that we only check for the
environment variable once.  If it is not set, we will fall back to the
cache directory, and mkdir() it to make sure that it exists.

Meanwhile, remove a statement from the reference documentation that
promises a warning in this case (which has never been true) and replace
it with a statement that applications can rely on the directory
existing.

This change prevents each user of this API from having to check for the
directory for themselves; an example of that can be seen in bug 763274.

https://bugzilla.gnome.org/show_bug.cgi?id=763344