Done some work on task #1549 (function documentation)

src/api/err.c

@@ -51,7 +51,12 @@ static const char *err_strerr[] = {
            "Timeout."                /* ERR_TIMEOUT -13 */
 };
 
-
+/**
+ * Convert an lwip internal error to a string representation.
+ *
+ * @param err an lwip internal err_t
+ * @return a string representation for err
+ */
 const char *
 lwip_strerr(err_t err)
 {
@@ -59,5 +64,4 @@ lwip_strerr(err_t err)
 
 }
 
-
 #endif /* LWIP_DEBUG */