sys/fmt: Improve API documentation summary

sys/include/fmt.h

@@ -7,10 +7,22 @@
  */
 
 /**
- * @defgroup    sys_fmt string formatting
+ * @defgroup    sys_fmt String formatting (fmt)
  * @ingroup     sys
  * @brief       Provides simple string formatting functions
  *
+ * The goal of this API is to provide a string formatting interface which has a
+ * reduced code size footprint compared to the libc provided stdio.h functionality.
+ *
+ * This library provides a set of formatting and printing functions for 64 bit
+ * integers, even when the C library was built without support for 64 bit
+ * formatting (newlib-nano).
+ *
+ * \note The print functions in this library do not buffer any output.
+ * Mixing calls to standard @c printf from stdio.h with the @c print_xxx
+ * functions in fmt, especially on the same output line, may cause garbled
+ * output.
+ *
  * @{
  *
  * @file
@@ -269,7 +281,7 @@ void print_u64_hex(uint64_t val);
 /**
  * @brief Print uint64 value as decimal to stdout
  *
- * @note This used fmt_u64_dec(), which uses ~400b of code.
+ * @note This uses fmt_u64_dec(), which uses ~400b of code.
  *
  * @param[in]   val  Value to print
  */