RFR: 8326578: Clean up networking properties documentation

[Daniel Jeliński]

Please review this patch that removes the specification of system properties that are no longer used:
- `http.auth.digest.cnonceRepeat` system property was removed in JDK 5
- `java.net.socks.username` and `java.net.socks.password` were never available as system properties; they were available as user preferences. The support for these preferences was removed in JDK 7.

Additionally I updated the SOCKS proxy specification to match the user guide, and added the information that SOCKS proxies are only used by java.net TCP connections; the NIO SocketChannel does not support SOCKS proxies.

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Commit messages:
 - Remove documentation for removed properties

Changes: https://git.openjdk.org/jdk/pull/17988/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17988&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326578
  Stats: 25 lines in 1 file changed: 0 ins; 17 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/17988.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17988/head:pull/17988

PR: https://git.openjdk.org/jdk/pull/17988