Way to expand all cells of a help doc page at once?

While (on a Mac) Cmd+' works to toggle cell groups open and closed in notebooks, it doesn't work in the help documentation. There you have to instead use Cmd+Shift+[ (or, in Windows or Linux, Ctrl+Shift+[ ).

So to get it to work on all of the groups use Cmd/Ctrl+A then Cmd/Ctrl+Shift+[

Here's a way to do it programmatically in case that's ever useful:

FrontEndExecute@{FrontEndToken[InputNotebook[], "SelectAll"],
  FrontEndToken[InputNotebook[], "SelectionOpenAllGroups"]}

Or you can bind it to a button:

Button["Open Groups", 
 FrontEndExecute@{FrontEndToken[InputNotebook[], "SelectAll"], 
   FrontEndToken[InputNotebook[], "SelectionOpenAllGroups"]}
 ]

You could also bind it to the basic Cmd+' event:

{
 {"MenuCommand", 
   "OpenCloseGroup"} :> {FrontEndToken[InputNotebook[], "SelectAll"], 
   FrontEndToken[InputNotebook[], "SelectionOpenAllGroups"]}
 }

Attach that to the NotebookEventActions at the stylesheet level for the "Wolfram/Reference.nb" stylesheet (making sure to make a copy in your $UserBaseDirectory to edit).