diff --git a/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel b/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
--- a/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
+++ b/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel
@@ -28,6 +28,12 @@
 ]
 
 # A command-line flag for restricting the list of LLVM targets to build.
+#
+# See `llvm_targets` in `:targets.bzl` for the full list of available targets.
+#
+# Usage:
+#
+# $ bazel build --@llvm-projectllvm:targets=AArch64,X86 --config=generic_clang @llvm-project...
 string_list_flag(
     name = "targets",
     build_setting_default = llvm_targets,
@@ -43,7 +49,7 @@
     for target in llvm_targets
 ]
 
-# Replace macros in headers with lists of enabled targets.
+# Expand LLVM target enumerations in C++ headers with lists of enabled targets.
 [
     enum_targets_gen(
         name = name,
diff --git a/utils/bazel/llvm-project-overlay/llvm/enum_targets_gen.bzl b/utils/bazel/llvm-project-overlay/llvm/enum_targets_gen.bzl
--- a/utils/bazel/llvm-project-overlay/llvm/enum_targets_gen.bzl
+++ b/utils/bazel/llvm-project-overlay/llvm/enum_targets_gen.bzl
@@ -53,7 +53,9 @@
         ),
         "valid_targets": attr.string_list(
             mandatory = True,
-            doc = "A list of LLVM targets that are valid for this enum.",
+            doc = "A list of LLVM targets that are valid for this enum. " +
+                  "Since this value depends on what files are present, it " +
+                  "must be evaluated outside the implementation of the rule.",
         ),
         "targets": attr.label(
             mandatory = True,
diff --git a/utils/bazel/llvm-project-overlay/llvm/target_macros.bzl b/utils/bazel/llvm-project-overlay/llvm/target_macros.bzl
--- a/utils/bazel/llvm-project-overlay/llvm/target_macros.bzl
+++ b/utils/bazel/llvm-project-overlay/llvm/target_macros.bzl
@@ -17,10 +17,10 @@
     return [m.split("/")[2] for m in matches]
 
 def enabled_targets(pattern):
-    """Build a list of Bazel targets matching the pattern.
+    """Build a list of enabled LLVM targets.
 
-    This macro builds an expression that evaluates to a list of enabled
-    Bazel targets of the form `<target><pattern>`.
+    This macro builds a depency list that evaluates to Bazel targets of the
+    form `<target><pattern>` for every enabled LLVM target.
     """
     # Build a list of selects that either choose the Bazel target or not,
     # depending on the `:targets` flag.
@@ -32,7 +32,7 @@
         for t in llvm_targets
     ]
     # Fold over the selects with "+".
-    result = selects[0]
-    for s in selects[1:]:
+    result = []
+    for s in selects:
       result += s
     return result