[foreign-memaccess+abi] RFR: 8291286: Improve JavaDocs for static constructors in MemorySession

[Per-Åke Minborg]

This PR adds information to the static constructor methods in MemorySession to improve clarity for new users.

This is my first PR for a JDK project so I am eager to receive feedback and improvement suggestions from the community.

-------------

Commit messages:
 - Merge branch 'openjdk:foreign-memaccess+abi' into develop
 - Update after PR comments
 - Improve JavaDocs for static constructors in MemorySession

Changes: https://git.openjdk.org/panama-foreign/pull/693/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=693&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291286
  Stats: 42 lines in 1 file changed: 28 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/693.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/693/head:pull/693

PR: https://git.openjdk.org/panama-foreign/pull/693