Update blocks-management.rst

contain more articulated kind of contents ==> kinds of content
A Block come always with an editor ==> comes always
able to handled its content ==> to handle 
the Blocks menu remains active ==> the Block's menu
change the editor just clicking on => by just clicking


 by the red arrow an the picture => on the picture
under the current highlighted ==> currently highlighted

block has been choosen in ==> chosen

but the default hypertext editor let you edit ==> lets your

 block which can contain singles ==> contain single and/or included