doc: improve help message with regards to how filters are applied

[rolinh]

hubble help observe currently provides the following help message:

Observe provides visibility into flow information on the network and
application level. Rich filtering enable observing specific flows related to
individual pods, services, TCP connections, DNS queries, HTTP requests and
more.

Usage:
  hubble observe [flags]
(...)

Something crucial that is missing from the help message is an explanation about how various filters are combined together.

@michi-covalent provided a good explanation in this PR comment. It could be adapted/extended to explain how this applies to hubble observe also with extended information about how the --not filter works in combination of the others.