drm/i915: Improve debug Kconfig texts a bit

drm/i915: Improve debug Kconfig texts a bit

We're not consistently recommending these for developers only.

I stumbled over this due to DRM_I915_LOW_LEVEL_TRACEPOINTS, which was
added in

commit cd69238acf8ef8ed9e46927e7849bc6f1d217821
Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Date:   Tue Feb 21 11:01:42 2017 +0000

    drm/i915/tracepoints: Add request submit and execute tracepoints

to "alleviate the performance impact concerns."

Which is nonsense.

Tvrtko and Joonas pointed out on irc that the real (but undocumented
reason) was stable abi concerns for tracepoints, see

https://lwn.net/Articles/705270/

and the specific change that was blocked around tracepoints:

https://lwn.net/Articles/442113/

Anyway to make it a notch clearer why we have this Kconfig option
consistly add the "Recommended for driver developers only." to it and
all the other debug options we have.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702201708.2075793-1-daniel.vetter@ffwll.ch