Improve Language Tour to communicate that `if` guard clause can't take functions

[erikareads]

The current language tour doesn't communicate that you can't use functions in a guard clause: https://gleam.run/book/tour/case-expressions.html#checking-equality-and-ordering-in-pattern

I think there's an implicit "only" here, that should be made explicit:

The guard expression can check for equality or ordering for Int and Float.

[erikareads]

The Try Gleam tour also doesn't yet talk about guard clauses in Flow Control, when you add that section it would be worth mentioning that functions can't be used.

[lpil]

Thank you