RFR: 8037384: Fix wording in Javadoc of java.io.Serializable [v2]

[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.

Stuart Marks has updated the pull request incrementally with one additional commit since the last revision:

  non-serializable classes => non-serializable superclasses

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1306/files
  - new: https://git.openjdk.java.net/jdk/pull/1306/files/ce7e6d6c..54c17ba6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1306&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1306&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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