Sounds like a great idea! Just hit this too. It looks like the fix is trivial (adding a no-op `doc` parameter), so I've mailed https://github.com/bazelbuild/bazel/pull/6936 . Note that rules using this param won't work with any older Bazel versions.
`repository_rule` should have a `doc` argument
`repository_rule` is similar to `rule`, except it lives on the `WORKSPACE` level. `rule` has a nice `doc` attribute which is extracted by documentation tools. It would be nice if `repository_rule` followed suit.

In [`rules_nixpkgs`](https://github.com/tweag/rules_nixpkgs) and [`rules_haskell`](https://github.com/tweag/rules_haskell) we have to resort to [manually written docs](https://github.com/tweag/rules_nixpkgs/blob/master/README.md#rules-1), which tend go out of sync easily.
src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryModule.java src/main/java/com/google/devtools/build/lib/skylarkbuildapi/repository/RepositoryModuleApi.java src/main/java/com/google/devtools/build/skydoc/fakebuildapi/repository/FakeRepositoryModule.java
src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryModule.java src/main/java/com/google/devtools/build/lib/skylarkbuildapi/repository/RepositoryModuleApi.java src/main/java/com/google/devtools/build/skydoc/fakebuildapi/repository/FakeRepositoryModule.java
src/test/java/com/google/devtools/build/skydoc/testdata/misc_apis_test/input.bzl
diff --git a/src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryModule.java b/src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryModule.java
index a1532dad894b31..61e3884706789c 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryModule.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryModule.java
@@ -60,6 +60,7 @@ public BaseFunction repositoryRule(
       Object attrs,
       Boolean local,
       SkylarkList<String> environ,
+      String doc,
       FuncallExpression ast,
       com.google.devtools.build.lib.syntax.Environment funcallEnv)
       throws EvalException {
diff --git a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/repository/RepositoryModuleApi.java b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/repository/RepositoryModuleApi.java
index 36728cb4856864..b41304bfbdb5e7 100644
--- a/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/repository/RepositoryModuleApi.java
+++ b/src/main/java/com/google/devtools/build/lib/skylarkbuildapi/repository/RepositoryModuleApi.java
@@ -78,6 +78,15 @@ public interface RepositoryModuleApi {
                     + "an environment variable in that list change, the repository will be "
                     + "refetched.",
             named = true,
+            positional = false),
+        @Param(
+            name = "doc",
+            type = String.class,
+            defaultValue = "''",
+            doc =
+                "A description of the repository rule that can be extracted by documentation "
+                    + "generating tools.",
+            named = true,
             positional = false)
       },
       useAst = true,
@@ -87,6 +96,7 @@ public BaseFunction repositoryRule(
       Object attrs,
       Boolean local,
       SkylarkList<String> environ,
+      String doc, 
       FuncallExpression ast,
       Environment env)
       throws EvalException;
diff --git a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/repository/FakeRepositoryModule.java b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/repository/FakeRepositoryModule.java
index 8f22cb7585ca11..8f72a4ebbb52f8 100644
--- a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/repository/FakeRepositoryModule.java
+++ b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/repository/FakeRepositoryModule.java
@@ -27,7 +27,7 @@ public class FakeRepositoryModule implements RepositoryModuleApi {
 
   @Override
   public BaseFunction repositoryRule(BaseFunction implementation, Object attrs, Boolean local,
-      SkylarkList<String> environ, FuncallExpression ast, Environment env) {
+      SkylarkList<String> environ, String doc, FuncallExpression ast, Environment env) {
     return implementation;
   }
 }

diff --git a/src/test/java/com/google/devtools/build/skydoc/testdata/misc_apis_test/input.bzl b/src/test/java/com/google/devtools/build/skydoc/testdata/misc_apis_test/input.bzl
index d083028376cf84..24a75a703bd7d2 100644
--- a/src/test/java/com/google/devtools/build/skydoc/testdata/misc_apis_test/input.bzl
+++ b/src/test/java/com/google/devtools/build/skydoc/testdata/misc_apis_test/input.bzl
@@ -8,7 +8,10 @@ def my_rule_impl(ctx):
 def exercise_the_api():
     var1 = config_common.FeatureFlagInfo
     var2 = platform_common.TemplateVariableInfo
-    var3 = repository_rule(implementation = my_rule_impl)
+    var3 = repository_rule(
+        implementation = my_rule_impl,
+        doc = "This repository rule has documentation.",
+    )
     var4 = testing.ExecutionInfo({})
 
 exercise_the_api()
