doc: Merged/updated old docs for Element.extend
doc: Merged/updated old docs for Element.extend

fashionsun/prototype,lamsieuquay/prototype,Gargaj/prototype,ldf7801528/prototype,baiyanghese/prototype,loduis/prototype,sstephenson/prototype,lamsieuquay/prototype,ridixcr/prototype,ShefronYudy/prototype,sdumitriu/prototype,leafo/prototype,fashionsun/prototype,erpframework/prototype,292388900/prototype,baiyanghese/prototype,loduis/prototype,ldf7801528/prototype,Gargaj/prototype,ShefronYudy/prototype,Jiasm/prototype,ridixcr/prototype,Gargaj/prototype,leafo/prototype,sstephenson/prototype,Jiasm/prototype,fashionsun/prototype,ridixcr/prototype,292388900/prototype,loduis/prototype,292388900/prototype,sdumitriu/prototype,Jiasm/prototype,lamsieuquay/prototype,sstephenson/prototype,ShefronYudy/prototype,ldf7801528/prototype,erpframework/prototype,sdumitriu/prototype,erpframework/prototype