doc/go1.18: document restrictions for real, imag, complex

For #47694.
For #50912.
For #50937.

Change-Id: I3fae6c8dbbd61a45e669b8fb0c18ac76f2183963
Reviewed-on: https://go-review.googlesource.com/c/go/+/381967


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>