Improve Launchpadlib documentation

Improve the documentation of Launchpadlib, it is a Wiki (and I have already created a user account for it).

Ideas for improvement of Launchpadlib documentation
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These ideas are for the main help document which is linked to in various places:
https://help.launchpad.net/API/launchpadlib

Describe model of Launchpadlib: Objects on Launchpad appear as interlinked
Python objects, that are "magically" downloaded to your computer. However your
changes to those objects are not automatically uploaded to Launchpad, nor are
the objects automatically updated if they change on the server.

Therefore...
Document methods ``lp_save``.
Document method ``lp_refresh``.

Explain the representation of bugs in Launchpad, which is very different to
other bug trackers. Bugs are represented by two entities: "bug" and "bug-task".

In what timezone are the time values? UTC probably.