[foreign-memaccess+abi] RFR: 8305369: Issues in zero-length memory segment javadoc section

[Maurizio Cimadamore]

This patch addresses a number of issue in the zero-length memory segment javadoc section:

* there are some typo in the code samples (e.g. extra semi-colons);
* variable names are off (e.g. spurious references to a `foreign` segment variable);
* the explanation of how a pointer is read from a segment is lacking, and could be clarified;
* it would be useful to have links to other section (e.g. in the Linker javadoc) which also deal with ZLMS.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/825/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=825&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305369
  Stats: 45 lines in 2 files changed: 11 ins; 8 del; 26 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/825.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/825/head:pull/825

PR: https://git.openjdk.org/panama-foreign/pull/825