doc/dev: edit "Principles for format change"

doc/dev: edit "Principles for format change"

Improve "Principles for format change" in doc/dev/encoding.rst. This
commit started as a response to Anthony D'Atri's suggestion here: https://github.com/ceph/ceph/pull/58299/files#r1656985564

Review of this section suggested to me that certain minor English usage
improvements would be of benefit. The numbered lists in this section
could still be made a bit clearer.

Co-authored-by: Anthony D'Atri <anthony.datri@gmail.com>
Signed-off-by: Zac Dover <zac.dover@proton.me>