RFR: 8338728: Misc issues in memory layout javadoc

[Maurizio Cimadamore]

This PR fixes two minor issues in the `MemoryLayout` javadoc:
* the section describing dereference path talks about `P` and `P'` but then only uses `P` in the code;
* the `ceilDiv` math on the `PathElement::sequenceElement(long, long)` is wrong, as the division returns a negative number for F < 0, which is incorrect

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jdk/pull/20659/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20659&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338728
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/20659.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20659/head:pull/20659

PR: https://git.openjdk.org/jdk/pull/20659