objectcache: Document the primary entry points people should use

objectcache: Document the primary entry points people should use

There are imho too many ObjectCache methods, $wg*Cache* variables,
CACHE_ types, and wfGet*Cache() functions to know which ones
should actually be used publicly. This should make it easier to
know which ones should be used.

The difference between them was also harder to understand with the
main documentation sections about each of these living in
conceptually different places:
- classes (WANObjectCache)
- methods (ObjectCache::getMainStashInstance)
- variables (DefaultSettings: wgMainCacheType, wgMainWANCache, wgMainStash)

Also:
* Correct wfGetCache() documentation.
* Add the missing keywords for 'public' visibility.

Change-Id: I3b05fdb8b7888bf7e3f05bdca36538f3484556b8