Add non-zero requirement for conio and DMA copy

A DMA copy/fill count of 0 actually means 64 kB.
This is an issue for e.g. conio and empty strings.
This adds documentation to address this.

include/mega65/conio.h

@@ -912,8 +912,9 @@ void fastcall cputs(const unsigned char* s);
  * @brief Output multiple screen codes at X,Y coordinates
  * @param x The X coordinate where string will be printed
  * @param y The Y coordinate where string will be printed
- * @param s An array of screen codes to print
+ * @param s An array of screen codes to print. Must have non-zero length.
  * @remarks This function works with screen codes only. To output ordinary
+ * @warning Undefined behavior if `s` has zero length.
  */
 void cputsxy(unsigned char x, unsigned char y, const unsigned char* s);
 
@@ -943,8 +944,9 @@ void cputcxy(unsigned char x, unsigned char y, unsigned char c);
  * @brief Output N copies of a single character at X,Y coordinates
  * @param x The X coordinate where character will be printed
  * @param y The Y coordinate where character will be printed
- * @param count The number of characters to output
+ * @param count The number of characters to output. Must be larger than zero.
  * @param c The screen code of the characters to print
+ * @warning Undefined behavior if `count` is zero.
  */
 void cputncxy(
     unsigned char x, unsigned char y, unsigned char count, unsigned char c);

include/mega65/memory.h

@@ -141,23 +141,23 @@ void dma_poke(uint32_t address, uint8_t value);
  * @brief Copy a block of memory using DMA
  * @param source_address 28-bit address to copy from
  * @param destination_address 28-bit address to copy to
- * @param count Number of bytes to copy
+ * @param count Number of bytes to copy. Note that 0 = 64 kB.
  */
 void lcopy(uint32_t source_address, uint32_t destination_address, size_t count);
 
 /**
  * @brief Fill a block of memory with a single byte using DMA
  * @param destination_address Start address (28-bit)
  * @param value Fill value
- * @param count Number of bytes to fill
+ * @param count Number of bytes to fill. Note that 0 = 64 kB.
  */
 void lfill(uint32_t destination_address, uint8_t value, size_t count);
 
 /**
  * @brief Fill a block of memory with a single byte using DMA with a step
  * @param destination_address Start address (28-bit)
  * @param value Fill value
- * @param count Number of bytes to fill
+ * @param count Number of bytes to fill. Note that 0 = 64 kB.
  * @param skip Skip every n bytes
  */
 void lfill_skip(