RFR: 8371950: The jdk.httpclient.keepalive.timeout system networking property does not specify the behaviour for values <= 0

[Daniel Fuchs]

This change adds a sentence to the the description of the three keepalive properties to describe the effect of setting a value <= 0:

> If the provided value is negative, the default value is used. A value of 0 causes the timeout to fire immediately. 

I also took this opportunity to add a link from `module-info.java` to the `java.net.http` package description, and an `@see` link from the `java.net.http` package description to the `module-info`.

This is a doc change only - there is no code or behaviour change. Since it only affects jdk specific properties and is just a clarification of pre-existing behavior I believe no CSR is required. Please comment if you believe writing a CSR would be better.

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Commit messages:
 - 8371950: The jdk.httpclient.keepalive.timeout system networking property does not specify the behaviour for values <= 0

Changes: https://git.openjdk.org/jdk/pull/29587/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371950
  Stats: 11 lines in 2 files changed: 8 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/29587.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29587/head:pull/29587

PR: https://git.openjdk.org/jdk/pull/29587