scripts/kernel-doc: Add support for named variable macro arguments

Currently, when kernel-doc encounters a macro with a named variable
argument[1], such as this:

   #define hlist_for_each_entry_rcu(pos, head, member, cond...)

... it expects the variable argument to be documented as `cond...`,
rather than `cond`. This is semantically wrong, because the name (as
used in the macro body) is actually `cond`.

With this patch, kernel-doc will accept the name without dots (`cond`
in the example above) in doc comments, and warn if the name with dots
(`cond...`) is used and verbose mode[2] is enabled.

The support for the `cond...` syntax can be removed later, when the
documentation of all such macros has been switched to the new syntax.

Testing this patch on top of v5.4-rc6, `make htmldocs` shows a few
changes in log output and HTML output:

 1) The following warnings[3] are eliminated:

   ./include/linux/rculist.h:374: warning:
        Excess function parameter 'cond' description in 'list_for_each_entry_rcu'
   ./include/linux/rculist.h:651: warning:
        Excess function parameter 'cond' description in 'hlist_for_each_entry_rcu'

 2) For list_for_each_entry_rcu and hlist_for_each_entry_rcu, the
    correct description is shown

 3) Named variable arguments are shown without dots

[1]: https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
[2]: scripts/kernel-doc -v
[3]: See also https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/commit/?h=dev&id=5bc4bc0d6153617eabde275285b7b5a8137fdf3c



Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>