Document SI for JSON after initial testing

[glookka]

Bug Description:

After we test/fix the secondary indexes for json attributes feature, we need to document it

Manticore Search Version:

6.3.1

Operating System Version:

Any

Have you tried the latest development version?

Yes

Internal Checklist:

^{To be completed by the assignee. Check off tasks that have been completed or are not applicable.}

• [x] Implementation completed
• [x] Tests developed
• [x] Documentation updated
• [ ] Documentation reviewed
• [x] Changelog updated