Release process: document setting version manually

Originally created by @CyrilBrulebois on #17578 (Redmine)

As life happens, preparing for all possible use cases isn’t sufficient!

Here’s a proposal that was floated around when discussing version numbers vs. changelog entries (see also #17577 (closed)), and that I’ve confirmed to be something I’d be very much interested in:

>> IMO we should set $VERSION manually, instead of assuming the changelog
>> knows what version we're releasing.                                   
>                                                                        
> I think we should be documenting that explicitly, yes.                 
>                                                                        
> 1 week, 2 use cases:                                                   
>  - emergency release: need to move from 4.(n+1) to 4.n.1               
>  - release candidate: need to move from 4.5 to 4.5~rc1                 

Related issues

• Related to #17361 (closed)