clear clang errors

src/string/string.h

@@ -13,12 +13,12 @@
  *
  * In either case, the string is reallocated to match the length before
  * returning.
- * @param filename
+ * @param filename a complete pathname to the file
  * @param nread if not null, this will have the total amount bytes read
  * @param nmax if not 0, this amount of memory in bytes is read and used as
  * initial allocation
  */
-const char *hi_string_from_file_dyn(const char *filename, size_t *nread,
+char *hi_string_from_file_dyn(const char *filename, size_t *nread,
                                     size_t nmax);
 
 /**