Clarify explanations and defaults in diaspora.yml.example

In this PR I have attempted to clarify how to use this file, by:

- stating the defaults for each setting
- clarifying the explanation of each setting
- making sure the setting in the commented line is *not* the same as the default
- altering the order of items to match that in defaults.yml

Please check you're happy with what I've done, in particular check that I haven't added any factual inaccuracies.

There are several points I'm not sure of:

- 'Use max-age header on Amazon S3 resources' - does this mean that setting this to false will mean that  assets will be deleted after 1 year, or that the default (true) does this? Or something else entirely?
- 'Related to S3: set a url to redirect all requests to uploaded images to another host' - I don't get this. Is this to set an *alternative* assets host to using Amazon S3, or a redirect *as part of* Amazon S3 hosting?
- 'Write unicorn stderr and stdout log' - does this need any explanation as to what it does and in what circumstances it might be useful?
- If you enable community spotlight, do you *have* to set an email address to which suggestions can be sent, or is this optional?