ReleaseHotFix.md should encourage branches from release

The docs/ReleaseHotFix.md documentation suggests that branches that are intended to be backported to the release branch should be branched off of master. However, based off of a conversation between @kmorel and @ben.boeckel on !2763 (comment 1182032), a more reasonable mechanism would be to branch the topic branch off of release and then merge to master and backport to release.

Given this, the documentation should be change to describe the topic branch being branched off of release. At the end of the documentation should be some advice if the topic branch is already off of master and conflicts with release (such as git rebase master --onto release).