RFR: 8326578: Clean up networking properties documentation [v3]

[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.

Daniel Jeliński has updated the pull request incrementally with one additional commit since the last revision:

  Clarify default ProxySelector

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17988/files
  - new: https://git.openjdk.org/jdk/pull/17988/files/321cb672..1cf4345b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17988&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17988&range=01-02

  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 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