Convert @stage to short form

This CL converts remaining @stage instances in the dawn tree to use
the equivalent shorter variant.

Bug: tint:1503
Change-Id: I74594cd68544fbd692f77d4646991d9c27e218f8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/92484
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

docs/tint/spirv-input-output-variables.md

@@ -94,7 +94,7 @@ Current translation, through SPIR-V, SPIR-V reader, WGSL writer:
       return;
     }
 
-    @stage(fragment)
+    @fragment
     fn main() -> void {
       bar_();
       return;
@@ -126,7 +126,7 @@ Proposed translation, through SPIR-V, SPIR-V reader, WGSL writer:
       @location(0) frag_color : vec4<f32>;
     };
 
-    @stage(fragment)
+    @fragment
     fn main(
 
       // 'in' variables are entry point parameters