Bug 1055374: Clean up Debugger.Memory's documentation, and link to it from the main Debugger API page. DONTBUILD r=jorendorff

Now the page begins with summaries of Debugger.Memory's features, with
references to specific accessors and methods; and then a reference section that
describes them in detail below.

Remove references to path-finding and retained size computation, which aren't
implemented yet.