fix: encoder handling WriteSeeker

Unnexpected "encoder internal error" occurs when passing io.WriteSeeker as io.Writer since we forgot to early return. Previously, we treat an io.Writer that implements both io.WriterAt and io.WriteSeeker, such as *os.File, as io.WriterAt so users using *os.File does not affected by this bug. The bug only affects io.Writer that only implement io.WriteSeeker.
Now io.Writer that implements both io.WriterAt and io.WriteSeeker will be treated as io.WriteSeeker. We change this to prevent wrong write on a io.Writer that is not empty (existing file) as we will aways write at offset 0 to update the header while offset 0 might not be our own content. By treating it as io.WriteSeeker, we can seek relative to EOF so we ensure that we write on our own data. This also enable us to append to existing file to create a chained FIT file.
Add caveats documentation when users are using io.Writer that solely implements io.WriterAt must provide an empty writer.
In most cases, we encode to a brand new file, so the bug is unlikely to occur broadly.

muktihari / fit