modernize contains_eager() docs

modernize contains_eager() docs

Along with other loader options that are likely
to require usage of populate_existing(), make sure
contains_eager() documentation makes it absolutely
clear that already-loaded collections are not overwritten.
consolidate contains_eager() documentation into the narrative docs.

Additionally, remove the "arbitrary statements" section, this is
not a useful case and seems to be left over.

Fixes: #5740
Change-Id: I10e320882990f511eefebcc88cfcc2277e553b50
(cherry picked from commit 0d76857ee1022f41baffff4091be765deea4e421)