Make documentation less vague on length of Change IDs

[udf2457]

As presently worded, the documentation is very vague on the actual length of Change IDs.

In relation to change IDs it states:

They are typically 16 bytes long

typically is an odd choice of terminology ? It raises two paths:

• If they are 16 bytes, then just say so ?
• If they can change in length (and importantly, if they can increase in length) then the docs should state when/how because people might be building external tooling.