Mention new scopes for if and while

[sogaiu]

In a Zulip discussion about the if special form, it came up that the Special Forms page doesn't appear to mention that if and while can introduce new lexical scopes (though the coverage for do and fn touch on the topic).

This PR suggests the addition of some text to remedy the situation.

[pepe]

LGTM

[rwtolbert]

I think when does the same as if.

[sogaiu]

Yes, I think when does.

IIUC, that is because it's a macro that expands to if:

$ janet
Janet 1.37.1-60d9f977 linux/x64/gcc - '(doc)' for help
repl:1:> (macex1 '(when true :hello))
(if true (do :hello))

However, this PR is specifically about the current Special Forms page and I think because when is a macro (and not a special form), it's not covered there.