Fix wrong := REPL documentation (#39975)

Author: Liozou

stdlib/REPL/src/docview.jl

@@ -220,7 +220,7 @@ function lookup_doc(ex)
     if isa(ex, Symbol) && Base.isoperator(ex)
         str = string(ex)
         isdotted = startswith(str, ".")
-        if endswith(str, "=") && Base.operator_precedence(ex) == Base.prec_assignment
+        if endswith(str, "=") && Base.operator_precedence(ex) == Base.prec_assignment && ex !== :(:=)
             op = str[1:end-1]
             eq = isdotted ? ".=" : "="
             return Markdown.parse("`x $op= y` is a synonym for `x $eq x $op y`")

stdlib/REPL/test/repl.jl

@@ -1092,6 +1092,9 @@ end
 @test occursin("identical", sprint(show, help_result("===")))
 @test occursin("broadcast", sprint(show, help_result(".<=")))
 
+# Issue 39427
+@test occursin("does not exist", sprint(show, help_result(":=")))
+
 # Issue #25930
 
 # Brief and extended docs (issue #25930)