framework: add missing methods to interface and docs

The set-replace-start and get-replace-search-hit methods were
missing in the text:searchable<%> interface. The latter was
also undocumented.

collects/framework/private/text.rkt

@@ -861,6 +861,8 @@
 
 (define searching<%> 
   (interface (editor:keymap<%> basic<%>)
+    set-replace-start
+    get-replace-search-hit
     set-searching-state
     set-search-anchor
     get-search-bubbles

collects/scribblings/framework/text.scrbl

@@ -360,7 +360,12 @@
     found last time that a search happened.
   }
 
-  @defmethod[(set-replace-start [pos (or/c false/c number?)]) void?]{
+  @defmethod[(get-replace-search-hit) (or/c number? #f)]{
+    Returns the position of the nearest search hit that comes after the
+    position set by the @method[text:searching<%> set-replace-start] method.
+  }
+
+  @defmethod[(set-replace-start [pos (or/c number? #f)]) void?]{
     Sets the position where replacement next occurs. This is equivalent to
     calling @method[text:searching<%> set-searching-state] with a new
     @racket[replace-start] argument, but the other arguments the same as the