Update string.h.md

doc/api/mruby/string.h.md

+## Macros
+### mrb_str_ptr(s)
+Returns a pointer from a Ruby string.
+## Functions
 ### mrb_str_plus
 ```C
    mrb_value mrb_str_plus(mrb_state*, mrb_value, mrb_value);
@@ -43,3 +47,45 @@ Duplicates a string object.
    mrb_value mrb_str_intern(mrb_state *mrb, mrb_value self);
 ```
 Returns a symbol from a passed in string.
+### mrb_str_to_str
+```C
+   mrb_value mrb_str_to_str(mrb_state *mrb, mrb_value str);
+```
+Returns a converted string type.
+### mrb_str_equal
+```C
+   mrb_bool mrb_str_equal(mrb_state *mrb, mrb_value str1, mrb_value str2);
+```
+Returns true if the strings match and false if the strings don't match.
+### mrb_str_cat
+```C
+   mrb_value mrb_str_cat(mrb_state *mrb, mrb_value str, const char *ptr, size_t len);
+```
+Returns a concated string comprised of a Ruby string and a C string.
+### mrb_str_cat_cstr
+```C
+   mrb_value mrb_str_cat_str(mrb_state *mrb, mrb_value str, mrb_value str2);
+```
+Returns a concated string comprised of a Ruby string and a C string(A shorter alternative to mrb_str_cat).
+### mrb_str_append
+```C
+   mrb_value mrb_str_append(mrb_state *mrb, mrb_value str, mrb_value str2);
+```
+Adds str2 to the end of str1.
+### mrb_str_cmp
+```C
+   int mrb_str_cmp(mrb_state *mrb, mrb_value str1, mrb_value str2);
+```
+Returns 0 if both Ruby strings are equal.
+Returns a value < 0 if Ruby str1 is less than Ruby str2.
+Returns a value > 0 if Ruby str2 is greater than Ruby str1.
+### mrb_str_to_cstr
+```C
+   char *mrb_str_to_cstr(mrb_state *mrb, mrb_value str);
+```
+Returns a C string from a Ruby string.
+### mrb_str_inspect
+```C
+   mrb_str_inspect(mrb_state *mrb, mrb_value str);
+```
+Returns a printable version of str, surrounded by quote marks, with special characters escaped.