[foreign-abi] RFR: 8253373: Add package-level javadoc for foreign linker support

[Maurizio Cimadamore]

This patch modifies the toplevel javadoc of jdk.incubator.foreign to add details about the ABI support.

I've decided to split the javadoc into two main sections - one about memory access, and one about foreign function
access; they have subsections for more detailed info.

There's also a trailing toplevel section on "restricted method". I'm still wordsmithing the definition of "what's it
like to be a restricted method" together with Alex, but this is a first attempt. It is likely that, once we have this
done, we can have the various restrcited definitions inside the API refer bakc to this section.

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Commit messages:
 - Add toplevel package javadoc for abi support

Changes: https://git.openjdk.java.net/panama-foreign/pull/346/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=346&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253373
  Stats: 142 lines in 1 file changed: 138 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/346.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/346/head:pull/346

PR: https://git.openjdk.java.net/panama-foreign/pull/346