[DOC] Add doc to sharing_detection= [ci skip]

Before:
```
$ ri sharing_detection=
= .sharing_detection=
(from ruby core)
=== Implementation from PP
------------------------------------------------------------------------
  sharing_detection=(b)
------------------------------------------------------------------------
Returns the sharing detection flag as a boolean value. It is false by
default.
```

After:
```
$ ri sharing_detection=
= .sharing_detection=

(from ruby core)
=== Implementation from PP
------------------------------------------------------------------------
  sharing_detection=(b)

------------------------------------------------------------------------

Sets the sharing detection flag to b.
```

lib/pp.rb

@@ -100,6 +100,7 @@ class PP < PrettyPrint
       def sharing_detection
         Ractor.current[:pp_sharing_detection]
       end
+      # Sets the sharing detection flag to b.
       def sharing_detection=(b)
         Ractor.current[:pp_sharing_detection] = b
       end