Minor corrections for ALTER TYPE ADD VALUE IF NOT EXISTS patch.

Produce a NOTICE when the label already exists, for consistency with other
CREATE IF NOT EXISTS commands.  Also, fix the code so it produces something
more user-friendly than an index violation when the label already exists.
This not incidentally enables making a regression test that the previous
patch didn't make for fear of exposing an unpredictable OID in the results.
Also some wordsmithing on the documentation.

src/include/nodes/parsenodes.h

@@ -2306,7 +2306,7 @@ typedef struct AlterEnumStmt
 	char	   *newVal;			/* new enum value's name */
 	char	   *newValNeighbor; /* neighboring enum value, if specified */
 	bool		newValIsAfter;	/* place new enum value after neighbor? */
-	bool        skipIfExists;   /* ignore statement if label already exists */
+	bool		skipIfExists;	/* no error if label already exists */
 } AlterEnumStmt;
 
 /* ----------------------