Update string.h.md

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.