Update user_documentation.rst after some setup experiences with different clients.

I have found that using the endings .vcf and .ics in the address book and calendar file names has been crucial for successful setup. This might be a bug with Radicale or with the clients I tried.
Unfortunately I did not get Evolution to work with radicale :(
Merge pull request #44 from Perseid/patch-1

Update user_documentation.rst after some setup experiences with differen...