Added no-else-return message example (#6265)

Author: matusvalo

doc/data/messages/n/no-else-return/bad.py

@@ -0,0 +1,7 @@
+def compare_numbers(a: int, b: int) -> int:
+    if a == b:  # [no-else-return]
+        return 0
+    elif a < b:
+        return -1
+    else:
+        return 1

doc/data/messages/n/no-else-return/good.py

@@ -0,0 +1,6 @@
+def compare_numbers(a: int, b: int) -> int:
+    if a == b:
+        return 0
+    if a < b:
+        return -1
+    return 1