15.1/relnotes: Organize Upgrading section

[vladlen]

Summary

Changes in installation.adoc:

1. Added a full pkgbase upgrade procedure. It is based on the

instructions from the BETA2/RC2 announcements. However, I am not
entirely sure whether the exact commands pkg upgrade
differ between BETA/RC and the final RELEASE. Please check.

2. Added a detailed boot loader upgrade section (UEFI and Legacy

BIOS). This text was taken verbatim from the FreeBSD 14.0
release notes, where it was already accurate and complete.

3. In the existing freebsd-update section, added a paragraph that

reminds Legacy BIOS users to manually update the boot loader.
(freebsd-update only updates the UEFI boot loader automatically.)

Changes in relnotes.adoc:

1. The "Upgrading FreeBSD" section was copied from the 15.0 release

notes. The pkgbase subsection was rewritten to point to the new
section in installation.adoc instead of keeping the old instructions,
as they were related to 15.0 only.

2. The "Introduction" section was also taken from the 15.0 release notes.

What is still missing:

• The "Upgrading FreeBSD" section should also include a list of known

incompatibilities and upgrade‑related issues. This needs to be
added.

[ziaee]

i was talking to @cperciva and there's too much duplication in these files. He proposed moving the upgrading instructions to 15.1R/upgrading.adoc, and i think we should.

[ziaee]

In D57421#1317704, @ziaee wrote:

i was talking to @cperciva and there's too much duplication in these files. He proposed moving the upgrading instructions to 15.1R/upgrading.adoc, and i think we should.

Ok I'm just going to iterate on it starting with this patch since the clock is ticking. The edits I suggested here I will apply in the commit separating the notes.

website/content/en/releases/15.1R/relnotes.adoc

60–68

69–81

85

[vladlen]

Significantly reworked version of the patch in https://reviews.freebsd.org/D57517 .