[jdk21u-dev] RFR: 8322321: Add man page doc for -XX:+VerifySharedSpaces

[Aleksey Shipilev]

Clean backport for parity with 21.0.3-oracle.

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Commit messages:
 - Backport f7dc257a206d3104d6d24c2079ef1fe349368c49

Changes: https://git.openjdk.org/jdk21u-dev/pull/120/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=120&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322321
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/120.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/120/head:pull/120

PR: https://git.openjdk.org/jdk21u-dev/pull/120