Clarify the validation of present associations
Clarify the validation of present associations

I think that it is not clear what means that _an association is
present_. Add that it is checking that the foreign key is not empty and
that the referenced object exists to clarify it.

georgeclaghorn/rails,yawboakye/rails,yalab/rails,vipulnsward/rails,joonyou/rails,mechanicles/rails,kmcphillips/rails,jeremy/rails,lcreid/rails,Vasfed/rails,shioyama/rails,yawboakye/rails,starknx/rails,Envek/rails,esparta/rails,rails/rails,kddeisz/rails,Stellenticket/rails,esparta/rails,utilum/rails,esparta/rails,joonyou/rails,rafaelfranca/omg-rails,rails/rails,yalab/rails,lcreid/rails,Envek/rails,rafaelfranca/omg-rails,fabianoleittes/rails,georgeclaghorn/rails,yhirano55/rails,kmcphillips/rails,tjschuck/rails,EmmaB/rails-1,yahonda/rails,utilum/rails,fabianoleittes/rails,flanger001/rails,tjschuck/rails,yhirano55/rails,mechanicles/rails,tjschuck/rails,tgxworld/rails,iainbeeston/rails,repinel/rails,BlakeWilliams/rails,jeremy/rails,esparta/rails,prathamesh-sonpatki/rails,Erol/rails,palkan/rails,schuetzm/rails,rails/rails,jeremy/rails,palkan/rails,tgxworld/rails,arunagw/rails,starknx/rails,arunagw/rails,betesh/rails,lcreid/rails,iainbeeston/rails,georgeclaghorn/rails,MSP-Greg/rails,Erol/rails,eileencodes/rails,fabianoleittes/rails,joonyou/rails,tgxworld/rails,arunagw/rails,Vasfed/rails,bogdanvlviv/rails,prathamesh-sonpatki/rails,rafaelfranca/omg-rails,palkan/rails,Edouard-chin/rails,lcreid/rails,iainbeeston/rails,starknx/rails,iainbeeston/rails,MSP-Greg/rails,MSP-Greg/rails,mechanicles/rails,schuetzm/rails,utilum/rails,bogdanvlviv/rails,shioyama/rails,eileencodes/rails,Stellenticket/rails,MSP-Greg/rails,mechanicles/rails,yhirano55/rails,shioyama/rails,BlakeWilliams/rails,bogdanvlviv/rails,utilum/rails,shioyama/rails,flanger001/rails,joonyou/rails,Erol/rails,schuetzm/rails,yawboakye/rails,yalab/rails,yahonda/rails,betesh/rails,Edouard-chin/rails,Stellenticket/rails,tgxworld/rails,arunagw/rails,BlakeWilliams/rails,georgeclaghorn/rails,kmcphillips/rails,yalab/rails,eileencodes/rails,kmcphillips/rails,Erol/rails,eileencodes/rails,Envek/rails,notapatch/rails,bogdanvlviv/rails,flanger001/rails,repinel/rails,notapatch/rails,kddeisz/rails,flanger001/rails,kddeisz/rails,notapatch/rails,kddeisz/rails,vipulnsward/rails,rails/rails,yhirano55/rails,prathamesh-sonpatki/rails,Envek/rails,gauravtiwari/rails,gauravtiwari/rails,gauravtiwari/rails,notapatch/rails,vipulnsward/rails,vipulnsward/rails,Edouard-chin/rails,Vasfed/rails,Vasfed/rails,yahonda/rails,tjschuck/rails,fabianoleittes/rails,repinel/rails,EmmaB/rails-1,Edouard-chin/rails,betesh/rails,palkan/rails,yawboakye/rails,betesh/rails,Stellenticket/rails,schuetzm/rails,repinel/rails,prathamesh-sonpatki/rails,jeremy/rails,yahonda/rails,BlakeWilliams/rails,EmmaB/rails-1