nir: Document the flatten/dont_flatten selection control options.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-By: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17921>

src/compiler/nir/nir.h

@@ -2813,7 +2813,17 @@ nir_block_last_phi_instr(nir_block *block)
 
 typedef enum {
    nir_selection_control_none = 0x0,
+
+   /**
+    * Defined by SPIR-V spec 3.22 "Selection Control".
+    * The application prefers to remove control flow.
+    */
    nir_selection_control_flatten = 0x1,
+
+   /**
+    * Defined by SPIR-V spec 3.22 "Selection Control".
+    * The application prefers to keep control flow.
+    */
    nir_selection_control_dont_flatten = 0x2,
 } nir_selection_control;