Added docstrings for pip_lock rule

rules/lock.bzl

@@ -6,7 +6,7 @@ def _get_path_relative_to_workspace(path, ctx):
     else:
         return paths.join(ctx.label.package, path)
 
-def _pip_lock(ctx):
+def _pip_lock_impl(ctx):
     requirements_txt_paths = " ".join([
         req_file.path for req_file in ctx.files.requirements
     ])
@@ -51,16 +51,60 @@ def _pip_lock(ctx):
     )]
 
 pip_lock = rule(
-    implementation = _pip_lock,
+    implementation = _pip_lock_impl,
+    doc = """
+        Defines a binary target that may be executed via `bazel run` in order to compile
+        any number of `requirements.txt` files into a single `requirements-lock.json`
+        file. This binary should be executed on all supported platforms to add the 
+        correct set of requirements to the lock file for each platform. 
+    """,
     attrs = {
-        "requirements": attr.label_list(allow_files = True),
-        "requirements_lock": attr.string(default = "requirements-lock.json"),
+        "requirements": attr.label_list(
+            allow_files = True,
+            doc = """
+                Files following the standard requirements file format 
+                (https://pip.pypa.io/en/stable/reference/pip_install/#requirements-file-format)
+
+                These should define direct dependencies only, and should not pin
+                dependency versions unless necessary.
+            """,
+        ),
+        "requirements_lock": attr.string(
+            default = "requirements-lock.json",
+            doc = """
+                A path relative to the package in which this rule is defined to which
+                the compiled lock file should be written. This file should 
+                be source-controlled and provided as input to the `pip_repository` rule.
+            """,
+        ),
         "python_version": attr.string(
             values = ["PY2", "PY3", "PY2AND3"],
             default = "PY2AND3",
+            doc = """
+                The Python versions for which to compile the requirement set. The
+                requirements lock file will contain one environment per Python version 
+                per platform. Each environment will define its locked requirement set.
+            """,
+        ),
+        "wheel_dir": attr.string(
+            default = "wheels",
+            doc = """
+                A path to a directory relative to the package in which this rule is
+                defined in which built wheels will be stored. If the given directory
+                does not already exist, it will be created. 
+                
+                Wheels will be built for any required distribution that is not already 
+                available as a wheel for the given environment. These wheels may be
+                committed to source control or published to a custom Python package 
+                index. If the latter approach is used, this binary should be run again
+                with the `index_url` attribute set to the URL of that index in order to 
+                resolve the new locations of those wheels.
+            """,
+        ),
+        "index_url": attr.string(
+            default = "https://pypi.org/simple",
+            doc = "The URL of a custom Python package index to use instead of PyPI.",
         ),
-        "wheel_dir": attr.string(default = "wheels"),
-        "index_url": attr.string(default = "https://pypi.org/simple"),
         "_lock_pip_requirements_py2": attr.label(
             default = "//src/bin:lock_pip_requirements_py2",
             cfg = "target",