eina_str: Extended documentation for eina_str_join and eina_str_join_len.

The parameter 'size' must take into account two extra bytes, otherwise:
---snip---
   char *str1 = "AAAA";
   char *str2 = "BBBB";
   size_t siz = strlen(str1)+strlen(str2);
   char *buf = calloc(1, siz);
   eina_str_join(buf, siz, '#', str1, str2);
   fprintf(stdout, "buf: %s\n", buf);
   free(buf);
---snip---

result is truncated:
---snip---
buf: AAAA#BB
---snip---