Update editor<%> docs to clarify that undo needs to be enabled (#49)

Author: alex-hhh

gui-doc/scribblings/gui/editor-intf.scrbl

@@ -30,6 +30,10 @@ Adds an undoer procedure to the editor's undo stack. If an undo is
  redoing) changes to an editor, and when this undoer is the first item
  on the undo (or redo) stack.
 
+Editor instances are created with no undo stack, so no undo operations
+ will be recorded unless @method[editor<%> set-max-undo-history] is
+ used to change the size of the undo stack.
+
 The system automatically installs undo records to undo built-in editor
  operations, such as inserts, deletes, and font changes.  Install an
  undoer only when it is necessary to maintain state or handle

gui-doc/scribblings/gui/editor-overview.scrbl

@@ -61,26 +61,30 @@ A brief example illustrates how editors work. To start, an editor
 ]
 
 At this point, the editor is fully functional: the user can type text
- into the editor, but no cut-and-paste operations are available. We
- can support all of the standard operations on an editor via the
- menu bar:
+ into the editor, but no cut-and-paste or undo operations are
+ available. We can support all of the standard operations on an editor
+ via the menu bar:
 
 @racketblock[
 (define mb (new menu-bar% [parent f]))
 (define m-edit (new menu% [label "Edit"] [parent mb]))
 (define m-font (new menu% [label "Font"] [parent mb]))
 (append-editor-operation-menu-items m-edit #f)
 (append-editor-font-menu-items m-font)
+(send t #,(:: editor<%> set-max-undo-history) 100)
 ]
 
-Now, the standard cut and paste operations work, and the user can even
- set font styles. The user can also insert an embedded editor by
- selecting @onscreen{Insert Text} from the @onscreen{Edit} menu; after
- selecting the menu item, a box appears in the editor with the caret
- inside. Typing with the caret in the box stretches the box as text is
- added, and font operations apply wherever the caret is active. Text
- on the outside of the box is rearranged as the box changes
- sizes. Note that the box itself can be copied and pasted.
+Now, the standard cut-and-paste operations work and so does undo, and
+ the user can even set font styles. The editor is created with no undo
+ history stack, @method[editor<%> set-max-undo-history] is used to set
+ a non-zero stack, so undo operations can be recorded. The user can
+ also insert an embedded editor by selecting @onscreen{Insert Text}
+ from the @onscreen{Edit} menu; after selecting the menu item, a box
+ appears in the editor with the caret inside. Typing with the caret in
+ the box stretches the box as text is added, and font operations apply
+ wherever the caret is active. Text on the outside of the box is
+ rearranged as the box changes sizes. Note that the box itself can be
+ copied and pasted.
 
 The content of an editor is made up of @defterm{@tech{snips}}. An
  embedded editor is a single snip from the embedding editor's