docs: Document meaning of ETags in developer docs

[dragotin]

As @butonic correctly states in #3988 (comment) we need to document the concept of the ETags (and possibly CTags in the future) in the developer documention.

References:

• etag propagation is not always synchronous #3988
• etag, oc-etag, ctag, sync-token and the future #3782
• https://github.com/cernbox/smashbox/blob/master/protocol/protocol.md#restrictions-and-limitations
• And a classic of the ownCloud literature: https://dragotin.wordpress.com/2015/03/13/owncloud-etags-and-fileids/

[stale]

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[kobergj]

etags now mentioned in devdocs. closing.