GX-20174 Update account documentation

[dominouk]

Summary

Updated documentation based on improved descriptions for the Number field of Company/Customer entities.

Note: This was a simple document update, without any API changes, so should not need to appear on the change-log, so I've not updated it. However if you insist on a change-log entry then please just let me know as I can add one.

Checklist

• [x] Documentation follows the Style Guide
• [x] JSON examples updated
• [x] Properties in JSON examples are in the same order as in property tables
• [x] Changelog dated the day when PR merged
• [x] Changelog accurately describes all changes
• [x] Changelog highlights the affected endpoints or operations
• [x] Changelog highlights any deprecations
• [x] All hyperlinks tested
• [x] Deprecation Table updated if any deprecations
• [x] SUMMARY.md updated if new pages added