Fix legacy comments on Token. Begin commenting the new ALTER TABLE RENAME

COLUMN code.  Fix a memory leak in the sqlite_rename_column() SQL function.

FossilOrigin-Name: 32edc8920376aabb84ebe1900eaa9512d23f1b44d6459e4916dc6b07db66e27c

src/sqliteInt.h

@@ -2281,9 +2281,11 @@ struct IndexSample {
 ** Each token coming out of the lexer is an instance of
 ** this structure.  Tokens are also used as part of an expression.
 **
-** Note if Token.z==0 then Token.dyn and Token.n are undefined and
-** may contain random values.  Do not make any assumptions about Token.dyn
-** and Token.n when Token.z==0.
+** The memory that "z" points to is owned by other objects.  Take care
+** that the owner of the "z" string does not deallocate the string before
+** the Token goes out of scope!  Very often, the "z" points to some place
+** in the middle of the Parse.zSql text.  But it might also point to a
+** static string.
 */
 struct Token {
   const char *z;     /* Text of the token.  Not NULL-terminated! */
@@ -3114,7 +3116,7 @@ struct Parse {
   With *pWith;              /* Current WITH clause, or NULL */
   With *pWithToFree;        /* Free this WITH object at the end of the parse */
 #ifndef SQLITE_OMIT_ALTERTABLE
-  RenameToken *pRename;
+  RenameToken *pRename;     /* Tokens subject to renaming by ALTER TABLE */
 #endif
 };