[llvm] [Docs] Incorporate the recent discussed changes to release process (PR #145285)

[Kiran Chandramohan via llvm-commits]

kiranchandramohan wrote:

> Move up the date 2 weeks

Since we are within a few weeks of the branch date, could you consider relaxing the criteria for RC1 (as an exception for LLVM 21 only) to accept any patch that would have gone into main?

https://github.com/llvm/llvm-project/pull/145285