Merge pull request #4333 from cgundogan/pr/newlib/doc

newlib/syscalls: minor doc fix

sys/newlib/syscalls.c

@@ -86,10 +86,10 @@ void _exit(int n)
  * @brief Allocate memory from the heap.
  *
  * The current heap implementation is very rudimentary, it is only able to allocate
- * memory. But it does not
- * - have any means to free memory again
+ * memory. But it does not have any means to free memory again
  *
- * @return [description]
+ * @return      pointer to the newly allocated memory on success
+ * @return      pointer set to address `-1` on failure
  */
 void *_sbrk_r(struct _reent *r, ptrdiff_t incr)
 {