[update] rework ETag usage

[BirgitBader]

(i) This issue has been manually transferred from a former internal repository, as a private repository issue cannot be transferred to a public repository.

Context

Follow up of #1193:

Goal is to describe/clarify the usage of ETags for:

cache management

• good match for strong/weak etags
• Care should be taken, which headers, embedding, etc are part of the ETag calculation

concurrency control

• might not be suitable, due to strong ETags being constraint to the entity's representation and not to it's logical state
• Problems with intermediate proxies, that convert strong to weak ETags
• Introduction of a separate token as an alternative

Reference

Details can be found in the discussion of #1193.