RFR: 8209038: Clarify the javadoc of Cipher.getParameters()

[Valerie Peng]

Anyone can help review this javadoc update? The main change is the wording for the method javadoc of Cipher.getParameters()/CipherSpi.engineGetParameters(). The original wording is somewhat restrictive and request is to broaden this to accommodate more scenarios such as when null can be returned.
The rest are minor things like add {@code } to class name and null, and remove redundant ".". 

Will file CSR after the review is close to being wrapped up.
Thanks~

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Commit messages:
 - 8209038: Clarify the javadoc of Cipher.getParameters()

Changes: https://git.openjdk.java.net/jdk/pull/8117/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8117&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8209038
  Stats: 254 lines in 2 files changed: 2 ins; 5 del; 247 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8117.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8117/head:pull/8117

PR: https://git.openjdk.java.net/jdk/pull/8117