Document MYPY_FORCE_COLOR and FORCE_COLOR in --help

Partially resolves #13815.

Only partially because the rst documentation should also mention this, which I haven't done yet because the natural home for that mention is in a page that we haven't created yet (imo). This commit also does not test this feature.

Author: wyattscarpenter

mypy/main.py

@@ -367,7 +367,10 @@ def infer_python_executable(options: Options, special_opts: argparse.Namespace)
 
 FOOTER: Final = """Environment variables:
   Define MYPYPATH for additional module search path entries.
-  Define MYPY_CACHE_DIR to override configuration cache_dir path."""
+  Define MYPY_CACHE_DIR to override configuration cache_dir path.
+  Define MYPY_FORCE_COLOR (or, failing that, FORCE_COLOR) to a non-zero integer
+  (or any non-empty non-integer string) to force mypy to output in color.
+  """
 
 
 class CapturableArgumentParser(argparse.ArgumentParser):