devtool: ide-sdk: trim _add_broken_srctree_prefix_map docstring

The docstring referenced test_devtool_ide_sdk_none_qemu and
test_devtool_ide_sdk_{code,none}_meson_clang, which no longer exist
since the previous commit merged the --ide=code/--ide=none tests. Point
at the current DevtoolIdeSdkGccTests/DevtoolIdeSdkClangTests classes
instead and shorten the docstring to the essentials.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>