DOCS-1018 - Update notes for legacy Cloud SIEM custom threat intelligence

[jpipkin1]

Purpose of this pull request

This pull request updates the notes at the top of the following articles:

• https://help.sumologic.com/docs/security/threat-intelligence/threat-indicators-in-cloud-siem/
• https://help.sumologic.com/docs/cse/administration/create-custom-threat-intel-source/

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

DOCS-1018