docs: Add horizontal spacers around filenames in custom toolchain guide (#2563)

This improves readability by clearly marking the beginnings and ends of
the three involved source files.

Follow-up of #2512, as discussed.

docs/toolchains.md

diff --git a/docs/toolchains.md b/docs/toolchains.md
index 2a59169..32f4a54 100644
--- a/docs/toolchains.md
+++ b/docs/toolchains.md
@@ -411,7 +411,10 @@
 Defining toolchains for this might look something like this:
 
 ```
+# -------------------------------------------------------
 # File: toolchain_impl/BUILD
+# Contains the tool definitions (runtime, headers, libs).
+# -------------------------------------------------------
 load("@rules_python//python:py_cc_toolchain.bzl", "py_cc_toolchain")
 load("@rules_python//python:py_exec_tools_toolchain.bzl", "py_exec_tools_toolchain")
 load("@rules_python//python:py_runtime.bzl", "py_runtime")
@@ -453,9 +456,11 @@
 cc_library(name = "headers", ...)
 cc_library(name = "libs", ...)
 
+# ------------------------------------------------------------------
 # File: toolchains/BUILD
 # Putting toolchain() calls in a separate package from the toolchain
-# implementations minimizes Bazel loading overhead
+# implementations minimizes Bazel loading overhead.
+# ------------------------------------------------------------------
 
 toolchain(
     name = "runtime_toolchain",
@@ -480,8 +485,10 @@
     exec_comaptible_with = ["@platforms/os:linux"]
 )
 
+# -----------------------------------------------
 # File: MODULE.bazel or WORKSPACE.bazel
-# These toolchains will considered before others
+# These toolchains will considered before others.
+# -----------------------------------------------
 register_toolchains("//toolchains:all")
 ```