preamble to Starlark rule documentation: mention label

Stardoc, as opposed to skydoc, ignores module doc-strings. Therefore,
add at least some basic information to the preamble that is added to
the generated markdown document; as that preamble is shared, we have
to keep the text generic, hence, for the time being, only mention the
label the files can be loaded from.

Fixes bazelbuild#9225.

Change-Id: I6b19cd8b5f68e9726c0a113a261e2064302152c1
PiperOrigin-RevId: 266356428

tools/build_defs/repo/BUILD

@@ -40,10 +40,17 @@ REPO_BZL_FILES = [
     deps = [":lib"],
 ) for name in REPO_BZL_FILES]
 
+[genrule(
+    name = "preamb_%s_md" % (name,),
+    srcs = [":preamb.md"],
+    outs = ["preamb_%s.md" % (name,)],
+    cmd = "sed 's/BZL_FILE_BASE_NAME/%s/g' < $< > $@" % (name,),
+) for name in REPO_BZL_FILES]
+
 [genrule(
     name = "%s_md" % (name,),
     srcs = [
-        ":preamb.md",
+        ":preamb_%s.md" % (name,),
         ":%s_raw_md" % (name,),
     ],
     outs = ["%s.md" % (name,)],

tools/build_defs/repo/preamb.md

@@ -2,3 +2,6 @@
 layout: documentation
 title: Repository rules
 ---
+
+The following functions can be loaded from
+`@bazel_tools//tools/build_defs/repo:BZL_FILE_BASE_NAME.bzl`.