[DOC] Update the document ID limit in the documentation

[ankitkala]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.

Issue 1: There is a limit of 512 bytes for Document ID but this isn't mentioned anywhere in the documentation anywhere.
Issue 2: This limit is not being honoured in the bulk updates (tracking bug)

We need to:

• Update the documentation with the _id limits (index data and bulk).
• Call out that (2.9 onwards), we're enforcing this limit during bulk updates as well.