gh-154746: Update docstring for collections.Counter (GH-154792)

Added a note to `collections.Counter`'s docstring that initializing a `Counter` from a `Mapping` or `Counter` will have different behaviour than initializing from other `Iterable`s.

This behaviour was already documented in the canonical docs but is missing from the help() output.

Author: brass75

Lib/collections/__init__.py

@@ -553,6 +553,9 @@ class Counter(dict):
     or multiset.  Elements are stored as dictionary keys and their counts
     are stored as dictionary values.
 
+    When constructed from a Mapping or Counter, the original object's
+    values will be used as the initial counts.
+
     >>> c = Counter('abcdeabcdabcaba')  # count elements from a string
 
     >>> c.most_common(3)                # three most common elements