Improve Generate weekly PR updates to post in Slack docs/process

[cahillsf]

Currently the release documentation for Generate weekly PR updates to post in Slack and help from the tool does not specify what the milestone option should correspond to

Based on the current behavior, the milestone should correspond to the branch the user has checked out if working off a previous release branch or the upcoming milestone if generating from main, because we are using the command: git rev-list HEAD to retrieve the commits.

Since HEAD refers to the currently checked-out branch's latest commit, it is either:

• retrieving all of the commits that have been cherry-picked to a previous release's branch (will correspond to that specific release milestone)
• retrieving all of the commits to main (will correspond to upcoming release milestone)

Currently the tool just uses the milestone to generate the link, but does not incorporate it when retrieving the commits, which has led to some inaccurate use of the tool

To improve this situation, we should do one or more of the following:

• update the documentation/help text to specify how the milestone option should be used
• improve the tool to work off 1 input to retrieve the commits and generate the milestone

/area release

[k8s-ci-robot]

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[cahillsf]

see this issue for context: #9918 (comment)

if takes this issue, they should be sure to solve for this as well, but the scope for the current issue is smaller and could be a quicker short term change

[typeid]

/assign

[typeid]

I have tried addressing this in #10092. Let me know what you think! :)