Add explicit section in regards to multiline expressions in let bindings

[nojaf]

Following up on the previous issue, I'd like to see a small section about what to do with multiline body expressions in let and member bindings.
Example:

let functionName x y = a

if a is long or multiline it should be placed on the next line with an indent.

let functionName x y = 
    a

regardless of what SynExpr a ends up being.

Some examples I consider bad:

let a = """
foobar, long string
"""

let b = async {
    return () // not following the other CE samples
}

let c = {
    Name = "Bilbo"
    Age = 112
}

let d = while f do
    printfn "%A" x

From my (technical) point of view, the only a few SynExpr that can start right after the = sign and it strikes me as inconsistent to have these.
The only one I could find in the current guide is:

let rec sizeLambda acc = function
    | Abs(x, body) -> sizeLambda (succ acc) body
    | App(lam1, lam2) -> sizeLambda (sizeLambda acc lam1) lam2
    | Var v -> succ acc

As MatchLambda is only 1 of 63 SynExpr cases, this just seems like not worth having.

[Smaug123]

In all cases I think you're correct and those are bad. I'll make a PR for that.