[style-guide] Format mutation expressions on their own line

I have a strong dislike of code where `->` and `<-` appear on the same line:

```fsharp
let f x =
let mutable keepGoing = false
match x with
| Some _ -> ()
| None -> keepGoing <- false
```

Because this code is relatively rare I think we should consider a style guide entry to explicitly say "put this on two lines"

```fsharp
let f x =
let mutable keepGoing = false
match x with
| Some _ ->
()
| None ->
keepGoing <- false
```

Start by reviewing the issue's 11-comment discussion, since no file or test is identified in the payload. Done means recording the proposed line-break rule in the project's style guide after the discussion reaches a decision.

Written by the indexing model from the issue text.