docs: reduce the amount of detail in the open/close port methods

[tonyandrewmeyer]

Simplify the open_port() and close_port() documentation:

• Link to the Juju documentation (soon to also be updated) to minimise the amount of syncing required
• Note that the behaviour differs between K8s sidecar and machine charms
• Remove the note that juju expose/juju unexpose is required (true for machine, not true for k8s sidecar)

Fixes #918

[benhoyt]

Changes look fine, just a typo, and the docs build seems to be failing (confirms my dislike of ReST).

[tonyandrewmeyer]

the docs build seems to be failing (confirms my dislike of ReST).

Ah, my bad. I built the docs after doing open_port() to make sure that I had the link syntax correct, and then lazily did not do that after making the close_port() change, and it's angry that the external link is there twice. Will fix.