docs: why replacePrefix2 is different

Qpf/Macro/NameUtils.lean

@@ -1,9 +1,16 @@
 open Lean Meta
 
 namespace Lean.Name
--- This function has diffreent behaviour from Name.replacePrefix
+/--
+  This function has different behaviour from Name.replacePrefix that can be viewed in the following example:
+
+  #eval replacePrefix  `a `b `a.b -- `a
+  #eval replacePrefix2 `a `b `a.b -- `b.b
+
+  This comes from the internal declaration of replacePrefix dropping the last named value before continuing
+-/
 def replacePrefix2 (old_pref new_pref : Name) : Name → Name
-  | Name.anonymous => Name.anonymous
+  | Name.anonymous => .anonymous
   | Name.str p s   => let p' := if p == old_pref then new_pref
                                 else replacePrefix2 old_pref new_pref p
                       Name.mkStr p' s