refactor: clarify %main% is runfiles-root-relative path (bazel-contrib#3537)

I had to go dig through the source to figure out if `%main%` was being
treated as
a runfiles-root or main-repo-runfiles relative path. Add this to the
variable
description so its unambiguous.

Author: rickeylev

python/private/stage2_bootstrap_template.py

@@ -27,7 +27,7 @@
 # ===== Template substitutions start =====
 # We just put them in one place so its easy to tell which are used.
 
-# Runfiles-relative path to the main Python source file.
+# Runfiles-root-relative path to the main Python source file.
 # Empty if MAIN_MODULE is used
 MAIN_PATH = "%main%"