[Enhancement]: Document Partial Cancellation Convention Where Balance Security is Same Security

[JSv4]

Description of Enhancement :

Apparently, there are situations where balance security after a partial cancellation has the same ID. Not clear how this should be handled in the documentation. Need to document.

Why is this Needed?

This could cause issues. Proposed that we just leave balance_security_id blank and make it clear that's the appropriate approach where there is a balance.

Anything else we need to know?

[jacobyavis]

Related to this row in the comments sheet attached to #507

AngelList	General questions / suggestions	Balance securities https://docs.google.com/document/d/13iXqVfq88uD4URAz2cPII2fzOTX-VVjYmXI58NmlBwY/edit		For uncertificated securities, balance securities may not be necessary - but for certificated securities, noted the balance approach may be required. For an event-driven ledger report, convention may need to remain Worth noting 'uncertificated securities' has no legal meaning under e.g. DE law