Document new behavior with respect to double hyphens

Double-hyphens on a command line are used to signal that all remaining arguments are positional arguments instead of options. This behavior matches POSIX behavior recommendations, cf. The GNU C Library: Program Argument Syntax Conventions.

Starting with CMake 3.20, CMake does not understand double-hyphens on the command line anymore (see the console output below). My expectation is that CMake continues to respect double-hyphens, in particular since there was no mention of this change in the release notes.

ubuntu@u2004:~/build$ cmake -- ../cmake-3.20.1
CMake Error: Unknown argument --
CMake Error: Run 'cmake --help' for all supported options.
ubuntu@u2004:~/build$ cmake --version
cmake version 3.20.1

CMake suite maintained and supported by Kitware (kitware.com/cmake).
ubuntu@u2004:~/build$ /usr/bin/cmake -- ../cmake-3.20.1
-- The C compiler identification is GNU 9.3.0
-- The CXX compiler identification is GNU 9.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
[snip]
ubuntu@u2004:~/build$ /usr/bin/cmake --version
cmake version 3.16.3

CMake suite maintained and supported by Kitware (kitware.com/cmake).