Patch #1601678: move intern() to sys.intern().

Doc/lib/libsys.tex

@@ -340,6 +340,21 @@ else:
   \versionadded{1.5.2}
 \end{datadesc}
 
+\begin{funcdesc}{intern}{string}
+  Enter \var{string} in the table of ``interned'' strings and return
+  the interned string -- which is \var{string} itself or a copy.
+  Interning strings is useful to gain a little performance on
+  dictionary lookup -- if the keys in a dictionary are interned, and
+  the lookup key is interned, the key comparisons (after hashing) can
+  be done by a pointer compare instead of a string compare.  Normally,
+  the names used in Python programs are automatically interned, and
+  the dictionaries used to hold module, class or instance attributes
+  have interned keys.  \versionchanged[Interned strings are not
+  immortal (like they used to be in Python 2.2 and before);
+  you must keep a reference to the return value of \function{intern()}
+  around to benefit from it]{2.3}
+\end{funcdesc}
+
 \begin{datadesc}{last_type}
 \dataline{last_value}
 \dataline{last_traceback}