RFR: 8367546: Clarify meaning of "corresponding header" for architecture-qualified .inline.hpp files

[Francesco Andreuzzi]

During the review of #27189 we stumbled on the lack of a clear indication of what constitutes the `corresponding .hpp` file for a `.inline.hpp` file having an architecture qualifier.

For example, `continuationHelper_x86.inline.hpp` includes `runtime/continuationHelper.hpp`, and the include statement is on top as stated in the style guide:


#ifndef CPU_X86_CONTINUATIONHELPER_X86_INLINE_HPP
#define CPU_X86_CONTINUATIONHELPER_X86_INLINE_HPP

#include "runtime/continuationHelper.hpp"

#include "runtime/continuationEntry.inline.hpp"
#include "runtime/frame.inline.hpp"
#include "runtime/registerMap.hpp"
[...]


However, this order is not respected by `SortIncludes.java`, which does not designate `runtime/continuationHelper.hpp` as the "corresponding `.hpp` file". If this concept was clarified in the style guide, we could improve `SortIncludes` accordingly.

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Commit messages:
 - change guide

Changes: https://git.openjdk.org/jdk/pull/27259/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27259&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367546
  Stats: 15 lines in 2 files changed: 9 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/27259.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27259/head:pull/27259

PR: https://git.openjdk.org/jdk/pull/27259