The initial implementation of reflect.StructOf in
https://golang.org/cl/9251 had a limitation that field names had to be
ASCII, which was later lifted by https://golang.org/cl/21777.  Remove
the out-of-date documentation disallowing UTF-8 field names.

Updates: #5748
Updates: #15064

Change-Id: I2c5bfea46bfd682449c6e847fc972a1a131f51b7
Reviewed-on: https://go-review.googlesource.com/23170Reviewed-by: David Crawshaw <crawshaw@golang.org>