Add explanations for all AllocationDeciders

Relates to #4380
Relates to #2483
Remove support for boost in copy_to field

Currently, boosting on `copy_to` is misleading and does not work as originally specified in #4520. Instead of boosting just the terms from the origin field, it boosts the whole destination field.  If two fields copy_to a third field, one with a boost of 2 and another with a boost of 3, all the terms in the third field end up with a boost of 6.  This was not the intention.

  The alternative: to store the boost in a payload for every term, results in poor performance and inflexibility. Instead, users should either (1) query the common field AND the field that requires boosting, or (2) the multi_match query will soon be able to perform term-centric cross-field matching that will allow per-field boosting at query time (coming in 1.1).
Don't load CompetionTerms if lookupFactory is null

Closes #4970
Closes #4788
Prevent usage of object based RamUsageEstimator in Completion090PostingsFormat
Add RamUsageEstimator#sizeOf(Object) to forbidden APIs

This method can be a performance trap since it traverse the
entire object tree that is referenced by the provided object.
See LUCENE-5373