[foreign-memaccess] RFR: Fix javadoc in jdk.incubator.foreign.Foreign

[Maurizio Cimadamore]

This simple patch fixes an issue in the Foreign javadoc which was referring to the concept of "primordial" segment (a
synonim of Nothing segment). As per recent changes, now we can speak of address that do _not_ have a segment (e.g.
address.segment() == null), so javadoc should be rectified to reflect that.

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Commit messages:
 - Fix javadoc in jdk.incubator.foreign.Foreign

Changes: https://git.openjdk.java.net/panama-foreign/pull/98/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/98/webrev.00
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/98.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/98/head:pull/98

PR: https://git.openjdk.java.net/panama-foreign/pull/98