Clarify & provide prescriptive guidance on using CDC settings

[exalate-issue-sync]

Kathryn Hancox (kathancox) commented:

Following some support issues, it might be worthwhile to be more prescriptive in the advice we provide on the changefeed settings (number/enabling)

• {{SET CLUSTER SETTING kv.rangefeed.separated_intent_scan.enabled = true}}
• turn +kv.rangefeed.catchup_scan_iterator_optimization.enabled setting+ +on+
• {{changefeed.backfill.concurrent_scan_requests}}: 64

Also take a look at the language around the concurrent_scan_requests to ensure this describes this setting per node and as multiplicative

cc @amruss

Jira Issue: DOC-2528