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

[Tobias Hieta via llvm-commits]

tru wrote:

> Since we are within a few weeks of the branch date, could you consider relaxing the criteria for RC1/RC2 (as an exception for LLVM 21 only) to accept any patch that would have gone into main?
>
> Edit: RC1/RC2 instead of RC1. RC2 is 2 weeks after branch date and hence is a better match for the previous schedule.

Fair point and I will consider this when merging fixes.

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