StructField.ByteSize is almost always unset; document that Type.Size()
is the place to look.

The dwarf package doesn't spend much effort teaching you DWARF, so I
don't know what level of handholding is appropriate. Still, no harm in a
little comment.

Closes #21093

Change-Id: I0ed8cad2fa18e10a47d264ff16c176d603d6033c
Reviewed-on: https://go-review.googlesource.com/71671
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>