Clarify semantics of the ghc_default_version module extension

extensions/ghc_version.bzl

@@ -1,4 +1,11 @@
-""" Module extension which configures the GHC version"""
+"""Module extension which configures the default GHC version for rules_haskell and rules_haskell_tests
+
+If used from rules_haskell or rules_haskell_tests, this will change the default
+GHC version to the value of the `GHC_VERSION` environment variable (if set).
+
+When used from other modules than rules_haskell and rules_haskell_tests, the
+default GHC version is given by the `DEFAULT_GHC_VERSION` constant (see [`//haskell:ghc.bzl`](haskell/ghc.bzl).
+"""
 
 load(
     "@rules_haskell//haskell:private/ghc_ci.bzl",