Clairfy the rule around the use of `return` in do-blocks

[omus]

The return keyword should always be omitted from return statements within short-form method definitions (f(...) = ...). The return keyword should never be omitted from return statements within any other context (function ... end, macro ... end, etc.).

I think it would be good to clarify/revisit the use of return statements in do-blocks. Although do-blocks are just functions there are used commonly like if if/let/begin blocks where the last sub-expression returned.

A particularly annoying example of requiring return statements in do-blocks can be found when working with sprint:

msg = sprint() do io
    Base.showerror(io, e)
    Base.show_backtrace(io, catch_backtrace())
    println(io)
    return nothing
end

Excluding the return statement reduces confusion where someone might think that msg would be assigned to nothing.