Add docs for HasMany saveStrategy.
Add docs for HasMany saveStrategy.

Refs cakephp/docs#3553

Phillaf/cakephp,elboletaire/cakephp,nrother/cakephp,gmponos/cakephp,HavokInspiration/cakephp,ADmad/cakephp,ADmad/cakephp,CakeDC/cakephp,cakephp-fr/cakephp,michaelze/cakephp,ADmad/cakephp,cakephp/cakephp,CVO-Technologies/cakephp,petteyg/cakephp,berrygoudswaard/cakephp,CakeDC/cakephp,petteyg/cakephp,bravo-kernel/cakephp,CakeDC/cakephp,Phillaf/cakephp,dakota/cakephp,petteyg/cakephp,garethellis36/cakephp,mvdriel/cakephp,petteyg/cakephp,timhoeppner/cakephp,tgr0ss/cakephp,nrother/cakephp,jeffblack360/cakephp,jeffblack360/cakephp,cakephp-fr/cakephp,justinyost/cakephp,elboletaire/cakephp,garethellis36/cakephp,tenkoma/cakephp,ionas/cakephp,thinkingmedia/cakephp,davidyell/cakephp,eryw/cakephp,gmponos/cakephp,cakephp/cakephp,ionas/cakephp,eryw/cakephp,Phillaf/cakephp,timhoeppner/cakephp,CakeDC/cakephp,nrother/cakephp,dakota/cakephp,bravo-kernel/cakephp,tgr0ss/cakephp,davidyell/cakephp,garethellis36/cakephp,elboletaire/cakephp,ADmad/cakephp,michaelze/cakephp,mvdriel/cakephp,eryw/cakephp,mvdriel/cakephp,davidyell/cakephp,timhoeppner/cakephp,mvdriel/cakephp,CVO-Technologies/cakephp,cakephp-fr/cakephp,garethellis36/cakephp,HavokInspiration/cakephp,cakephp/cakephp,tgr0ss/cakephp,ndm2/cakephp,tenkoma/cakephp,ndm2/cakephp,michaelze/cakephp,justinyost/cakephp,bravo-kernel/cakephp,HavokInspiration/cakephp,tenkoma/cakephp,dakota/cakephp,cakephp/cakephp,gmponos/cakephp,thinkingmedia/cakephp,jeffblack360/cakephp,thinkingmedia/cakephp,ndm2/cakephp,tenkoma/cakephp,berrygoudswaard/cakephp,ndm2/cakephp,dakota/cakephp,berrygoudswaard/cakephp,ionas/cakephp,justinyost/cakephp,CVO-Technologies/cakephp