DOC-11837 multiple backup service issues (#3559)

* Finished revisions to existing documentation.
Added section on sizing the number of Backup Service nodes to address DOC-11837

* Misc. edits and corrections.

* More edits and typo fixes

* Yet more last minute edits

* Update modules/learn/pages/services-and-indexes/services/backup-service.adoc

Co-authored-by: Matt Hall <matthew@quickbeam.me.uk>

* Update modules/learn/pages/services-and-indexes/services/backup-service.adoc

Co-authored-by: Matt Hall <matthew@quickbeam.me.uk>

* Updates based on Matt's feedback.

* Fixes suggested by Beth.

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Co-authored-by: Matt Hall <matthew@quickbeam.me.uk>

Author: ggray-cb