kconfig: add short explanation to SYMBOL_WRITE

replace the question mark in the comment after SYMBOL_WRITE with an explanation
Signed-off-by: NMartin Walch <walch.martin@web.de>
Signed-off-by: N"Yann E. MORIN" <yann.morin.1998@free.fr>

scripts/kconfig/expr.h

@@ -93,7 +93,7 @@ struct symbol {
 #define SYMBOL_CHOICEVAL  0x0020  /* used as a value in a choice block */
 #define SYMBOL_VALID      0x0080  /* set when symbol.curr is calculated */
 #define SYMBOL_OPTIONAL   0x0100  /* choice is optional - values can be 'n' */
-#define SYMBOL_WRITE      0x0200  /* ? */
+#define SYMBOL_WRITE      0x0200  /* write symbol to file (KCONFIG_CONFIG) */
 #define SYMBOL_CHANGED    0x0400  /* ? */
 #define SYMBOL_AUTO       0x1000  /* value from environment variable */
 #define SYMBOL_CHECKED    0x2000  /* used during dependency checking */