Update email.md to make it clear SMTP not on Cloud

[evolve2k]

Im using cloud instance and followed these docs and spent time trying to setup SMTP. Eventually I discovered that it cant be done on Cloud instance, per this forum post: https://akaunting.com/forum/discussion/general/smtp-on-cloud-instance

Updating the docs to make everyone's life easier.

[evolve2k]

It was frustrating to:

1. Search for the docs looking for ways I can maybe use my custom domain.
2. The docs imply that I can set SMTP
3. Browse the settings and find they dont match the docs and that the options I need arent there
4. Look for and find the forums, search and find that infact SMTP doesnt work on cloud instance.

This PR resolves the main issue in the docs that are misleading.