Update docs.md

[ailisp]

According to man

       int stat(const char *pathname, struct stat *statbuf);
       int fstat(int fd, struct stat *statbuf);
       int lstat(const char *pathname, struct stat *statbuf);
...
       int fstatat(int dirfd, const char *pathname, struct stat *statbuf,
                   int flags);

path_filestate_get should be closer to fstatat instead of stat