[foreign-abi] RFR: Misc javadoc fixes (take two)

[Maurizio Cimadamore]

Just realizd that in my pass over the API I forgot to double check FunctionDescriptor.
Also, I reverted a spurious old change to MemoryLayout, as well as some `protected` modifiers in some of the leaf
layout classes.

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Commit messages:
 - More API fixes

Changes: https://git.openjdk.java.net/panama-foreign/pull/374/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=374&range=00
  Stats: 24 lines in 4 files changed: 4 ins; 0 del; 20 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/374.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/374/head:pull/374

PR: https://git.openjdk.java.net/panama-foreign/pull/374