Merge pull request #2929 from Mav7/master

C string.h API documentation.

doc/api/mruby/string.h.md

+### mrb_str_plus
+```C
+   mrb_value mrb_str_plus(mrb_state*, mrb_value, mrb_value);
+```
+Adds to strings together.
+### mrb_ptr_to_str
+```C
+   mrb_value mrb_ptr_to_str(mrb_state *, void*);
+```
+Converts pointer into a Ruby string.
+### mrb_obj_as_string
+```C
+   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.