Small fixes in DocumentationView

1. Skip view update if user try to show already opened documentation. Old behavior:
   • Open some documentation.
   • Go to the search edit, erase last character and press 'Return'.
   • The page will be reloaded. Also history will be appended to the *same* page - it's ugly.

2. Sync current search edit text and it's completer prefix. Old version has buggy behavior:
   • Open some context-documentation *from editor* (not from documentation view).
   • Press 'Return' in search edit (don't change the text).
   • View will display another documentation page, according to the previous state of completer.