[foreign-abi] RFR: Tweak API and javadoc for VaList

[Maurizio Cimadamore]

This is a small patch which fixes some javadoc and API issues in the CSupport.VaList class. Main changes are:

* CSupport::newVaList has been moved as a static factory in VaList (new name is `make`)
* Valist::toAddress() renamed to just `address`
* minor javadoc tweaks to better clarify the distinction between underlying C va list and the Java VaList warpper

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Commit messages:
 - Tweak API and javadoc for VaList

Changes: https://git.openjdk.java.net/panama-foreign/pull/206/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/206/webrev.00
  Stats: 59 lines in 4 files changed: 26 ins; 4 del; 29 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/206.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/206/head:pull/206

PR: https://git.openjdk.java.net/panama-foreign/pull/206