[foreign-abi] RFR: 8253360: LibraryLookup javadoc should be clearer about how to create restricted segments

[Maurizio Cimadamore]

This patch adds a better description of how clients are supposed to derive segments from a lookup symbol, by stating
the need of keeping the symbol alive via segment attachment.

I also fixed a missing type parameter javadoc in the VaList API.

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Commit messages:
 - * Add more detailed javadoc on how to create restricted segment for global variables.

Changes: https://git.openjdk.java.net/panama-foreign/pull/345/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=345&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253360
  Stats: 11 lines in 2 files changed: 10 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/345.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/345/head:pull/345

PR: https://git.openjdk.java.net/panama-foreign/pull/345