[foreign-memaccess+abi] RFR: 8292034: Improve javadoc after memory segment/memory address unification [v3]

[Maurizio Cimadamore]

> This is a rather big patch which overhauls the javadoc of the `MemorySegment` class.
> A big thanks to Alex Buckley, who helped review the API javadoc and contributed many of the changes you see here.
> 
> The main things touched in this PR are:
> 
> * the section on unsafe segments is gone from the package-level javadoc. In its place, there's a new section on zero-length memory segments in the `MemorySegment` javadoc
> * the class javadoc for `MemorySegment` has been overhauled greatly. It now focusses on two kinds of segments (native segments, and heap segments) and define how address, size, alignment of both is handled by the API.
> * we introduce a distinction between memory segment and "the region of memory" which backs the segment, which helps spelling out a lot of these properties in a cleaner way
> * perhaps the biggest change in the javadoc is the section on memory segment alignment, which now is much more clearly defined, and provides lots of examples (as that's a complex topic).
> * some of the javadoc, in both this class and other classes have been tweaked to reflect the new terminology (for instance, references to the term `base address` are gone)
> 
> Other javadoc changes might follow, at some point later. The `Linker` class might need some attention, as we don't really spell out how zero-length segments interacts with downcalls and upcalls.
> But, given the size of the changes, I'd rather deal with that separately.

Maurizio Cimadamore has updated the pull request incrementally with two additional commits since the last revision:

 - Address more review comments
 - Address review comments

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/730/files
  - new: https://git.openjdk.org/panama-foreign/pull/730/files/fd36fcb7..5b02bbc7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=730&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=730&range=01-02

  Stats: 44 lines in 4 files changed: 0 ins; 2 del; 42 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/730.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/730/head:pull/730

PR: https://git.openjdk.org/panama-foreign/pull/730