mw.log: Fix appearance in jsdoc docs

• Set @ignore on the mw.log function, otherwise it conflicts with the mw.log namespace
• Make sure that mw.log.error is recognized as a method rather than a property
• Make mw.log.warn part of the mw.log namespace, rather than the mw namespace
• Remove mw.log-specific documentation from the header of the mw namespace
• Use @Example where needed

Bug: T352936
Change-Id: Ib3a32161d3ec16497e38ab09571e690141725ff1