In the process of creating a fix for issue #35321, I noticed what I believe to be a documentation omission. 

In Lib/importlib/_bootstrap.py the top level comment states that:

# IMPORTANT: Whenever making changes to this module, be sure to run
# a top-level make in order to get the frozen version of the module
# updated.

From my testing, it appears that the header file will only be updated when running `make regen-importlib`

To repo:
- make a code change in Lib/importlib/_bootstrap_external.py
- run a top-level `make`
- see that Python/importlib.h does not change
- run `make regen-importlib`
- see that Python/importlib.h has now been updated

The documentation in Lib/importlib/_bootstrap_external.py does in fact mention this.

I propose amending the documentation to include the correct instructions.

If this is deemed necessary, I will write the patch over the weekend.

I believe this is why Travis does a `make regen-all` to be sure everything is up to date.

A patch to keep instructions up-to-date would be nice, newcomers are not familiar with all `make regen-*` commands.

New changeset 1dc5cb9cb3447211069a7788208254b1cfa8ec98 by Brett Cannon (Nina Zakharenko) in branch 'master':
bpo-35887: Add make regen-importlib step to importlib._bootstrap docstring (GH-11777)
https://github.com/python/cpython/commit/1dc5cb9cb3447211069a7788208254b1cfa8ec98