Add missing doxygen comments

Change-Id: Ia4b1a5bc1d046633a7f1f91e23659d237d35e640
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/37842
Commit-Queue: Ben Clayton <bclayton@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
diff --git a/src/reader/spirv/parser_impl.h b/src/reader/spirv/parser_impl.h
index 579d681..c832a05 100644
--- a/src/reader/spirv/parser_impl.h
+++ b/src/reader/spirv/parser_impl.h
@@ -474,7 +474,11 @@
   /// @returns the instruction, or nullptr on error
   const spvtools::opt::Instruction* GetInstructionForTest(uint32_t id) const;
 
+  /// A map of SPIR-V identifiers to builtins
   using BuiltInsMap = std::unordered_map<uint32_t, SpvBuiltIn>;
+
+  /// @returns a map of builtins that should be ignored as they do not exist in
+  /// WGSL.
   const BuiltInsMap& ignored_builtins() const { return ignored_builtins_; }
 
  private: