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 354d036f
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

drivers/gpu/drm/i915/Kconfig.debug

@@ -207,6 +207,8 @@ config DRM_I915_LOW_LEVEL_TRACEPOINTS
 	  This provides the ability to precisely monitor engine utilisation
 	  and also analyze the request dependency resolving timeline.
 
+	  Recommended for driver developers only.
+
 	  If in doubt, say "N".
 
 config DRM_I915_DEBUG_VBLANK_EVADE
@@ -220,6 +222,8 @@ config DRM_I915_DEBUG_VBLANK_EVADE
 	  is exceeded, even if there isn't an actual risk of missing
 	  the vblank.
 
+	  Recommended for driver developers only.
+
 	  If in doubt, say "N".
 
 config DRM_I915_DEBUG_RUNTIME_PM
@@ -232,4 +236,6 @@ config DRM_I915_DEBUG_RUNTIME_PM
 	  runtime PM functionality. This may introduce overhead during
 	  driver loading, suspend and resume operations.
 
+	  Recommended for driver developers only.
+
 	  If in doubt, say "N"