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

[Per Minborg]

On Tue, 19 Dec 2023 14:29:21 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`

doc/panama_ffi.md line 209:

> 207: try (Arena arena = Arena.ofConfined()) {
> 208:     MemorySegment comparFunc = linker.upcallStub(comparHandle, comparDesc, arena);
> 209:     MemorySegment array = session.allocateFrom(0, 9, 3, 4, 6, 5, 1, 8, 2, 7);

I think it should read `arena.allocateFrom(...)`

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PR Review Comment: https://git.openjdk.org/panama-foreign/pull/914#discussion_r1432414067