scripts/mypy-all: Add a little comment documentation

scripts/mypy-all.sh

@@ -1,4 +1,6 @@
 #!/usr/bin/env bash
 #
+# Run mypy checks against all the relevant files
 
+# We assume that all `.py` files in git should be checked, and *only* those.
 mypy $@ $(git ls-tree --full-tree -r --name-only HEAD | grep -E '\.py$')