message: Update SharedTree storage document
diff: @@ -12,7 +12,7 @@ Existing Fluid DDSs generally assume that their documents will fit entirely in a
  > As presented above, the amount of the document that a client can view at once is limited by a client machine's memory. An optimization might expand that size to the amount of _disk_ space available to a client by using an on-disk cache (e.g. indexDB). But even with this orders-of-magnitude increase in available local space, it doesn't fundamentally change the approach for virtualization because a document might fit neither in a client's memory _nor_ its disk, either because the document is impressively huge or the disk is already close to full.
  -Allowing for partial downloads of the tree has implications on the layout of the storage data. For example, item 6 in the list above might not be necessary if the full tree was always available to a client, but for a client with only part of the tree, it needs that data locally available.
  +Allowing for partial downloads of the tree has implications on the layout and structure of the stored data, which this document will explore.
  > The downloaded region of the tree is always strictly within a client's [partial checkout](#glossary). A partial checkout is a client's "working space" restricted to some particular region of the tree. It is sanctioned by the server; the server knows that the client is restricted to activity in that area. The server can therefore enforce that the client downloads content in that area exclusively, and the server can filter the ops that it delivers to that client to be only the ones relevant to that area. This can provide the server a way to enforce permission boundaries for different parts of the tree as well as provide improved efficiency for clients since they will receive fewer ops. The implementation of partial checkouts itself is outside the scope of this document, but this document will provide the fundamental building blocks for such an implementation.
  