backup.sgml patch that adds information on custom format backups

Hello,

I'd like to submit the following patch that extends backup.sgml with a bit of practical but important information.

Project: postgresql
Patch filename: backup.sgml-cmd-v001.patch

The patch extends backup.sgml and adds practical information on custom format backups approach. Basically, we believe that plaintext backup format is suitable for a very limited range of use cases, and that in real world people are usually better off with a custom format backup. This is what we want PostgreSQL users to be aware of and provide some hands-on examples of how to do backups using this approach.

It is meant for application, and is against master branch.

The patch does pass 'make check' and 'make html' successfully.

PS: this is my first submission ever. So, if I'm missing something or not doing it as expected, please, do let me know. Thank you.

Ivan