update all docs

docs/lib2x-sub_str.md

@@ -0,0 +1,18 @@
+# sub_str
+
+The dir changed since 2.0. 
+
+Returns the part of the string from `begin` to `end`, or to the `end` of the string (`end` not included).
+
+## Parameters
+
+- `t` : The original string.
+- `begin` : The beginning index, inclusive.
+- `end` : The ending index, exclusive. If it's omitted, the substring begins with the character at the specified `begin` and extends to the end of the original string.
+
+## Examples
+
+    use <util/sub_str.scad>;
+    
+	echo(sub_str("helloworld", 0, 5)); // ECHO: "hello"
+	echo(sub_str("helloworld", 5));    // ECHO: "world"