docs/system/target-sparc: Improve the Sparc documentation

docs/system/target-sparc: Improve the Sparc documentation

Add some words about how to enable or disable boolean features,
and remove the note about a Linux kernel being available on the
QEMU website (they have been removed long ago already), and the
note about NetBSD and OpenBSD still having issues (they should
work fine nowadays).

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/2141
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240419084812.504779-4-thuth@redhat.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>