RFR: 8285827: Describe the keystore.pkcs12.legacy system property in the java.security file

[Weijun Wang]

We added a new system property back in https://bugs.openjdk.java.net/browse/JDK-8153005 but it's better to describe it in the `java.security` file as well.

Please review the text. I especially added the last sentence so that people won't set `-Dkeystore.pkcs12.legacy=false`.

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Commit messages:
 - add description

Changes: https://git.openjdk.java.net/jdk/pull/8452/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8452&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285827
  Stats: 13 lines in 1 file changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8452.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8452/head:pull/8452

PR: https://git.openjdk.java.net/jdk/pull/8452