Porting/release_managers_guide.pod: document version placeholders

[mauke]

---

• This set of changes does not require a perldelta entry.

[jkeenan]

Can we get someone who has done either a dev or production release to confirm the data in this documentation update? Thanks.

[thibaultduponchelle]

Can we get someone who has done either a dev or production release to confirm the data in this documentation update?

The documentation proposed by @mauke is indeed what is doing the make-rmg-checklist

I verified versions (after generation), POD syntax (non generated guide) and rendering (non generated guide).