RFR: 8037384: Fix wording in Javadoc of java.io.Serializable

[Stuart Marks]

8231547: Serializable class doc should link to serialization specification

Rewrite a couple confusing sentences in the Serializable class doc. This does affect normative text, but the edits are primarily to focus and clarify the text, not to make any semantic changes. Thus, a CSR request shouldn't be required for this change.

Also add and adjust some links and link markup to the Java Object Serialization Specification.

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Commit messages:
 - 8037384: Fix wording in Javadoc of java.io.Serializable

Changes: https://git.openjdk.java.net/jdk/pull/1306/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1306&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8037384
  Stats: 27 lines in 5 files changed: 5 ins; 1 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1306.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1306/head:pull/1306

PR: https://git.openjdk.java.net/jdk/pull/1306