Introduce (*SectionHeader32).fullName and add documentation comments.

Updates #15345

Change-Id: I8f3b8ab9492642d62e7aad010c91c68daea3f14b
Reviewed-on: https://go-review.googlesource.com/22301Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>