gdb/doc: be more verbose about .gdb_index changes in version 9

I had reason to read the docs relating to .gdb_index format, and ended
up having to look at historical versions of the documentation in order
to clarify exactly what changed with the introduction of version 9.

Given that we claim versions 7, 8, and 9 are all still allowable index
versions, I think our docs should make it clear what the current
format is, but also what previous formats looked like.

I did try to do the same for the changes from 7 to 8, but even after
reading commit 796a7ff8234cfaa8a I'm still not entirely clear what
changed.  If I gain some insight in the future then there might be a
follow on patch for that too.

Approved-By: Eli Zaretskii <eliz@gnu.org>

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