Refactor WikibasePage.get and overriding methods and improve documentation
Refactor WikibasePage.get and overriding methods and improve documentation

Document return values and add note to the method warning users
that modifying returned data could cause an undesired change
to the live content.

Also make use of dict.get and reorder declarations into
meaningful order (labels, descriptions, aliases, claims...).

Change-Id: I1bdefb6b4ed7789038adfd1b2e32138a56700d28

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core