[foreign-memaccess+abi] RFR: Refresh FFM documents [v2]

[Per Minborg]

On Wed, 20 Dec 2023 10:04:22 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This PR brings the FFM documents in sync with the finalized API in JDK 22. The most obvious changes are:
>> 
>> * in the foreign memory document, the section on structured access has been rectified, to take into account the extra offset parameter that is added to memory access var handles;
>> * in both documents, several calls to `allocateArray` and `allocateUtf8String` have been replaced with `allocateFrom`
>> * the section introducing the `Linker` in the ffi document has been revamped; it now talks more at length about the mapping between layouts and C types. The table with Linux/x64 mappings has been moved to a section in the appendix, and I have also added a mention of `Linker::canonicalLayouts`
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address review comments

Looks good! Maybe @JornVernee should have a look too?

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Marked as reviewed by pminborg (Committer).

PR Review: https://git.openjdk.org/panama-foreign/pull/914#pullrequestreview-1790707108