pythongh-128017: Make a note that sys variables are read-only (python#128887)

Author: srinivasreddy

Doc/library/sys.rst

@@ -8,7 +8,7 @@
 
 This module provides access to some variables used or maintained by the
 interpreter and to functions that interact strongly with the interpreter. It is
-always available.
+always available. Unless explicitly noted otherwise, all variables are read-only.
 
 
 .. data:: abiflags