sys,fmt: update and fix docu

sys/include/fmt.h

@@ -194,12 +194,13 @@ size_t fmt_s16_dec(char *out, int16_t val);
  * If @p out is NULL, will only return the number of bytes that would have
  * been written.
  *
+ * @pre fp_digits < 8 (TENMAP_SIZE)
+ *
  * @param[out] out          Pointer to the output buffer, or NULL
- * @param[in]  val          Fixed point value, MUST be <= 4
+ * @param[in]  val          Fixed point value
  * @param[in]  fp_digits    Number of digits after the decimal point
  *
  * @return      Length of the resulting string
- * @return      0 if @p fp_digits is > 4
  */
 size_t fmt_s16_dfp(char *out, int16_t val, unsigned fp_digits);
 
@@ -222,12 +223,13 @@ size_t fmt_s16_dfp(char *out, int16_t val, unsigned fp_digits);
  * If @p out is NULL, will only return the number of bytes that would have
  * been written.
  *
+ * @pre fp_digits < 8 (TENMAP_SIZE)
+ *
  * @param[out] out          Pointer to the output buffer, or NULL
  * @param[in]  val          Fixed point value
- * @param[in]  fp_digits    Number of digits after the decimal point, MUST be <= 9
+ * @param[in]  fp_digits    Number of digits after the decimal point
  *
  * @return      Length of the resulting string
- * @return      0 if @p fp_digits is > 9
  */
 size_t fmt_s32_dfp(char *out, int32_t val, unsigned fp_digits);
 
@@ -236,14 +238,18 @@ size_t fmt_s32_dfp(char *out, int32_t val, unsigned fp_digits);
  *
  * Converts float value @p f to string
  *
- * @pre -2^32 < f < 2^32
+ * If @p out is NULL, will only return the number of bytes that would have
+ * been written.
  *
- * @note This function is using floating point math. It pulls in about 2.4k
- *       bytes of code on ARM Cortex-M platforms.
+ * @attention This function is using floating point math.
+ *            It pulls in about 2.4k bytes of code on ARM Cortex-M platforms.
+ *
+ * @pre -2^32 < f < 2^32
+ * @pre precision < 8 (TENMAP_SIZE)
  *
  * @param[out]  out         string to write to (or NULL)
  * @param[in]   f           float value to convert
- * @param[in]   precision   number of digits after decimal point (<=7)
+ * @param[in]   precision   number of digits after decimal point
  *
  * @returns     nr of bytes the function did or would write to out
  */
@@ -340,18 +346,19 @@ void print_u64_dec(uint64_t val);
 /**
  * @brief Print float value
  *
+ * @note See fmt_float for code size warning!
+ *
  * @pre -2^32 < f < 2^32
+ * @pre precision < TENMAP_SIZE (== 8)
  *
  * @param[in]   f           float value to print
- * @param[in]   precision   number of digits after decimal point (<=7)
+ * @param[in]   precision   number of digits after decimal point
  */
 void print_float(float f, unsigned precision);
 
 /**
  * @brief Print null-terminated string to stdout
  *
- * @note See fmt_float for code size warning!
- *
  * @param[in]   str  Pointer to string to print
  */
 void print_str(const char* str);