Update string.h.md

doc/api/mruby/string.h.md

@@ -10,6 +10,36 @@ Adds to strings together.
 Converts pointer into a Ruby string.
 ### mrb_obj_as_string
 ```C
-   mrb_obj_as_string(mrb_state *mrb, mrb_value obj);
+   mrb_value mrb_obj_as_string(mrb_state *mrb, mrb_value obj);
 ```
 Returns an object as a Ruby string.
+### mrb_str_resize
+```C
+   mrb_value mrb_str_resize(mrb_state *mrb, mrb_value str, mrb_int len);
+```
+Resizes the string's length.
+### mrb_str_substr
+```C
+   mrb_value mrb_str_substr(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len);
+```
+Returns a sub string.
+### mrb_string_type
+```C
+   mrb_value mrb_string_type(mrb_state *mrb, mrb_value str);
+```
+Returns a Ruby string type.
+### mrb_str_new_cstr
+```C
+   const char *mrb_string_value_cstr(mrb_state *mrb, mrb_value *ptr);
+```
+Returns a Ruby string as a C string.
+### mrb_str_dup
+```C
+   mrb_value mrb_str_dup(mrb_state *mrb, mrb_value str);
+```
+Duplicates a string object.
+### mrb_str_intern
+```C
+   mrb_value mrb_str_intern(mrb_state *mrb, mrb_value self);
+```
+Returns a symbol from a passed in string.