lint-history: explain how TMPDIR can be used

Some users may want to take advantage of setting TMPDIR to another
location that might be faster for the linting process.

Reported-by: @ruv on GitHub
Signed-off-by: Elijah Newren <newren@gmail.com>

contrib/filter-repo-demos/lint-history

@@ -72,6 +72,14 @@ EXAMPLES
 
     To run eslint --fix on all .js files in history:
         lint-history --relevant 'return filename.endswith(b".js")' eslint --fix
+
+INTERNALS
+
+    Linting of files in history will be done by writing the files to a
+    temporary directory before running the linting program; the
+    location of this temporary directory can be controlled via the
+    TMPDIR environment variable as per
+    https://docs.python.org/3/library/tempfile.html#tempfile.mkdtemp.
     '''
 
 parser = argparse.ArgumentParser(description='Run a program (e.g. code formatter or linter) on files in history',