debug/dwarf: clarify StructField.ByteSize doc

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>

src/debug/dwarf/type.go

@@ -154,7 +154,7 @@ type StructField struct {
 	Name       string
 	Type       Type
 	ByteOffset int64
-	ByteSize   int64
+	ByteSize   int64 // usually zero; use Type.Size() for normal fields
 	BitOffset  int64 // within the ByteSize bytes at ByteOffset
 	BitSize    int64 // zero if not a bit field
 }