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

[Maurizio Cimadamore]

This patch refreshes the FFM API documents to reflect the latest API changes. There has been some reshuffling: I realized that we had two sections on unsafe memory segments, one at the end of the memory document, another at the beginning of the FFI document. Since unsafe segments are more common when interacting with native functions, I just kept the latter (and enhanced to explain the options in more details).

I also moved (and rewrote) the segment allocator section from the FFI doc to the memory document, as I think working with allocators and using custom arenas is more generally useful, even outside FFI support.

I also cleaned up a lot of minor incongruences in the code snippets, text and footnotes which still referred to old API names and concepts.

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Commit messages:
 - More whitespaces
 - Remove whitespaces and tabs
 - Drop spurious references to `Addressable`, `whileAlive`
 - Update FFM API docs

Changes: https://git.openjdk.org/panama-foreign/pull/805/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=805&range=00
  Stats: 182 lines in 2 files changed: 47 ins; 48 del; 87 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/805.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/805/head:pull/805

PR: https://git.openjdk.org/panama-foreign/pull/805