Improved spec for notifications

[glpatcern]

This PR clarifies how notifications are actually used by implementations, in particular Nextcloud.

• Added a full example and details of possible notification types
• Removed a required but unspecified field
• Added providerId (already present in Nextcloud's payload) as a required field. Without it the notification cannot be consumed by the target!
• Extended the description to cover for the expected/typical side effects as MAY behaviors following a notification

To be noted that the response HTTP code in case of success was/is 201 CREATED. It is debatable why such a response was chosen and whether a standard 200 OK or even 202 ACCEPTED (cf https://github.com/cs3org/OCM-API/issues/31) would be more appropriate, but this is left outside this PR as it would be a breaking change.

[glpatcern]

I'm moving forward and merging this to avoid leaving stuff behind. As explained, this is just adding comments and fixing the spec according to how it was actually implemented.