[doc/man3][OSSL_ENCODER] Move NOTES to the bottom

For consistency with `OSSL_DECODER.pod`, and `man-pages(7)`, the `NOTES`
section is moved at the end of the file.

According to `man-pages(7)` the recommended section order is:

> NAME
> SYNOPSIS
> CONFIGURATION      [Normally only in Section 4]
> DESCRIPTION
> OPTIONS            [Normally only in Sections 1, 8]
> EXIT STATUS        [Normally only in Sections 1, 8]
> RETURN VALUE       [Normally only in Sections 2, 3]
> ERRORS             [Typically only in Sections 2, 3]
> ENVIRONMENT
> FILES
> VERSIONS           [Normally only in Sections 2, 3]
> CONFORMING TO
> NOTES
> BUGS
> EXAMPLE
> SEE ALSO

This commit does not attempt to fix the order in all pages but focuses
only on `OSSL_ENCODER` which has a "twin" man page in `OSSL_DECODER`,
making the inconsistent section order quite jarring.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13932)
