[foreign-memaccess+abi] RFR: 8312059: Clarify the documention for variadic functions

[Jorn Vernee]

This patch attempts to refine the javadoc for variadic functions further. Particularly, it tries to make it clearer that the layouts the native linker rejects are derived from the C specification. Instead of trying to say which layouts are reject, we say which C types are rejected, and then give a hint about how to map those C types into layouts, using `canonicalLayouts()`, which would tell a client which layouts are rejected.

Additionally, this patch removes the mentions of prototype-less functions, which are not considered variadic functions in C. It is also not clear whether arguments passed to such a function should be treated as variadic or not in the ABI (see linked issue). Finally the latest C standard, C23, prototype-less functions no longer exist, and a declaration of the form `void foo()` instead means the same as `void foo(void)`. For these reason, passing arguments to prototype-less functions will also not be supported (officially) by the linker. I wasn't sure about whether to add a specific note about that. I figured that just not saying they are supported, combined with the state of the latest C standard, would make this obvious enough (?).

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Commit messages:
 - minor phrasing fix
 - re-add spurious brace removed
 - clarify variadic function docs in Linker

Changes: https://git.openjdk.org/panama-foreign/pull/846/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=846&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312059
  Stats: 21 lines in 1 file changed: 3 ins; 5 del; 13 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/846.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/846/head:pull/846

PR: https://git.openjdk.org/panama-foreign/pull/846