x86/vm86: Fix the misleading CONFIG_VM86 Kconfig help text

The CONFIG_VM86 Kconfig help text is actively misleading, so fix it:

  - Don't mark it 'obsolete' in the text as we'll support the ABI as long as CPUs
    support it.

  - Qualify the part about software emulation and mention that for some apps you
    want a real vm86 mode.

  - Don't scare users away from the option, instead explain what it does.

Reported-by: Stas Sergeev <stsp@list.ru>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>