scribble/manual: document `this-obj'

original commit: 3fb4561a628b375d2ead03b506f2a71edf9f0d06

collects/scribblings/scribble/manual.scrbl

@@ -945,6 +945,13 @@ not @racket[method-id].}
 Like @racket[method], but the hyperlink shows both the method name and
 the containing class/interface.}
 
+@defform[(this-obj)]{
+
+Within a @racket[defmethod] or similar form, typesets as a
+meta-variable that stands for the target of the method call. Use
+@racket[(this-obj)] to be more precise than prose such as ``this
+method's object.''}
+
 @; ------------------------------------------------------------------------
 @section[#:tag "doc-signatures"]{Documenting Signatures}