flash: add note about program size

Program size - the writable page size that flash controller supports.
Plus fix program page description - multiple pages program

drivers/FlashIAP.h

@@ -112,6 +112,7 @@ public:
 
     /** Get the program page size
      *
+     *  The page size defines the writable page size
      *  @return Size of a program page in bytes
      */
     uint32_t get_page_size() const;

hal/flash_api.h

@@ -75,9 +75,9 @@ int32_t flash_erase_sector(flash_t *obj, uint32_t address);
  */
 int32_t flash_read(flash_t *obj, uint32_t address, uint8_t *data, uint32_t size);
 
-/** Program one page starting at defined address
+/** Program pages starting at defined address
  *
- * The page should be at page boundary, should not cross multiple sectors.
+ * The pages should not cross multiple sectors.
  * This function does not do any check for address alignments or if size is aligned to a page size.
  * @param obj The flash object
  * @param address The sector starting address
@@ -97,6 +97,7 @@ uint32_t flash_get_sector_size(const flash_t *obj, uint32_t address);
 
 /** Get page size
  *
+ * The page size defines the writable page size
  * @param obj The flash object
  * @return The size of a page
  */