[foreign-abi] RFR: 8249803: Add LibraryLookup.Symbol (javadoc followup)

[Maurizio Cimadamore]

I realized that the javadoc for LibraryLookup was still MemoryAddress-centric. This patch rectifies that.

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Commit messages:
 - Another fix
 - Fix LibraryLookup javadoc

Changes: https://git.openjdk.java.net/panama-foreign/pull/259/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/259/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249803
  Stats: 6 lines in 1 file changed: 0 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/259.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/259/head:pull/259

PR: https://git.openjdk.java.net/panama-foreign/pull/259