Closes issue #22791: Improved datetime from timestamp methods documentation.

Original patch by Akira Li.
diff --git a/Lib/datetime.py b/Lib/datetime.py
index 2768e9b..de57472 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -1393,7 +1393,7 @@
 
     @classmethod
     def utcfromtimestamp(cls, t):
-        "Construct a UTC datetime from a POSIX timestamp (like time.time())."
+        """Construct a naive UTC datetime from a POSIX timestamp."""
         t, frac = divmod(t, 1.0)
         us = int(frac * 1e6)