Reflect prefixed map in uniqueId documentation

[chrisvxd]

This PR updates the documentation in uniqueId() to reflect the different counters used for each prefix by using clearly distinct numbers.

The current documentation implies that all prefixed IDs are incremented on the same counter, incrementing from uniqueId('contact_'); // contact_104 to uniqueId(); //105. This behaviour was changed in #3644 with the introduction of the prefix map, and so this documentation could be misleading to users.