Use @stopped_doc flag to preserve :stopdoc: across done_documenting reset

The previous approach (checking done_documenting == value) was too
broad and hid 501 items that should remain visible.

Instead, track :stopdoc: state with a dedicated @stopped_doc flag.
When done_documenting= resets document_self, it skips the reset if
@stopped_doc is set. The flag is cleared by :startdoc:.

This precisely preserves :stopdoc: directives without affecting
classes that need the done_documenting reset for file reopening.

Author: st0012

lib/rdoc/code_object.rb

@@ -125,6 +125,7 @@ def initialize_visibility # :nodoc:
     @received_nodoc      = false
     @ignored             = false
     @suppressed          = false
+    @stopped_doc         = false
     @track_visibility    = true
   end
 
@@ -204,9 +205,8 @@ def documented?
 
   def done_documenting=(value)
     return unless @track_visibility
-    return if @done_documenting == value
     @done_documenting  = value
-    @document_self     = !value
+    @document_self     = !value unless @stopped_doc
     @document_children = @document_self
   end
 
@@ -344,6 +344,7 @@ def start_doc
     @document_children = true
     @ignored    = false
     @suppressed = false
+    @stopped_doc = false
   end
 
   ##
@@ -354,6 +355,7 @@ def stop_doc
 
     @document_self = false
     @document_children = false
+    @stopped_doc = true
   end
 
   ##