RFR: 8328183: Minor mistakes in docs of PrintStream.append()

[Brian Burkhalter]

Clarify the behavior of `append` for a `null` `CharSequence` parameter and clean up a couple of other typos.

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Commit messages:
 - 8328183: Minor mistakes in docs of PrintStream.append()

Changes: https://git.openjdk.org/jdk/pull/18607/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18607&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328183
  Stats: 17 lines in 5 files changed: 5 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/18607.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18607/head:pull/18607

PR: https://git.openjdk.org/jdk/pull/18607