gallium/docs: clarify that samplers are not used with PIPE_BUFFER resources

Commit 8aba778fa2 "st/mesa: don't set
sampler states for TBOs" changed how texture buffer objects are handled.
Document the new convention.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/docs/source/context.rst

@@ -99,6 +99,14 @@ objects. They all follow simple, one-method binding calls, e.g.
   various debug messages, eventually reported via KHR_debug and
   similar mechanisms.
 
+Samplers
+^^^^^^^^
+
+pipe_sampler_state objects control how textures are sampled (coordinate
+wrap modes, interpolation modes, etc).  Note that samplers are not used
+for texture buffer objects.  That is, pipe_context::bind_sampler_views()
+will not bind a sampler if the corresponding sampler view refers to a
+PIPE_BUFFER resource.
 
 Sampler Views
 ^^^^^^^^^^^^^