fetchgit: nudge to update the manual when modifying parameters

Many parameters added over the past many years were not documented in the
manual. People likely simple didn't think to do that, so let's nudge them.

(cherry picked from commit ee97de3be9)

pkgs/build-support/fetchgit/default.nix

@@ -11,6 +11,8 @@
   in "${if matched == null then base else builtins.head matched}${appendShort}";
 in
 lib.makeOverridable (lib.fetchers.withNormalizedHash { } (
+# NOTE Please document parameter additions or changes in
+#   doc/build-helpers/fetchers.chapter.md
 { url, rev ? "HEAD", leaveDotGit ? deepClone
 , outputHash ? lib.fakeHash, outputHashAlgo ? null
 , fetchSubmodules ? true, deepClone ? false