Clarify what is the default value of `postfixId`

Editor assessment

Clarifying the default value of postfixId states exactly what the documentation now explains, and docs(microservices) types and scopes the change correctly. The subject tells the reader which option gained documentation and what aspect of it was clarified — the default — so no diff is needed to grasp the intent. No body follows, and a one-detail docs clarification does not require one. No footers appear. Hygiene is clean: imperative mood, no trailing period, sensible length, no pasted content. A small documentation improvement described precisely in a single line.