RMG: Simplify the corelist section

[thibaultduponchelle]

I am unhappy with the "Update Module::CoreList" sections.

Changes

• Move most common command first
• Make the section smaller
• Link to email thread with details on handling Module::CoreList for MAINT releases

---

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

[thibaultduponchelle]

@iabyn

[iabyn]

I can't really comment. The wording I added to the RMG 12 years ago just reflected that at the time, handling Module::CoreList was complex as it lived in all branches and needed to be consistently version-bumped. I can't remember the details, and I have no idea what (if anything) has changed or improved in the meantime.