Update manual to describe plain text editing

assets/manual/orgro-manual.org

@@ -243,12 +243,15 @@ way:
 
 * Editing
 
-Some limited editing features are available:
+Some limited “structured” editing features are available:
 
 - Tap a list item with a checkbox to toggle the checkbox
   - [ ] Try me!
 - Slide a section from right to left to cycle its TODO keyword
 
+In addition, plain text editing is available from the floating edit button in
+the bottom right corner.
+
 If Orgro has the appropriate permissions, it will save changes back to the
 original file. This is an experimental feature; see the [[https://orgro.org/faq/#can-i-edit-my-files-with-orgro][FAQ]] for details.