Add docs on using keyboard_post_init_user to enable debug modes

[zvecr]

Description

Now that #3113 has been merged, there is a nice programmatic way to toggle debug modes. This is useful when debugging matrix scan issues, where you cannot rely on keycodes/key presses.

Note: keyboard_pre_init_user still does not allow the same method, however i have this commit that could fix it. The main issue is that keyboard_pre_init_user runs before magic() in keyboard_init
setting anything to do with the debug_x variables in matrix_init_user gets reset by whats in eeconfig_read_debug a little later. Using matrix_init_kb (which also suffers this problem) looks like quite a common pattern in the keyboards folder, and these would all be broken... but i'm guessing they may have been from before the magic init code was introduced

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[drashna]

I think that using post_init here is better, actually.

But thanks!

[zvecr]

@drashna the same is true of matrix_init_kb, it can mean you miss some initialisation time logging. I would add a PR for the commit i previously mentioned, however a lot of keymaps and boards would suddenly have debug enabled. See split_common/matrix.c use of setting debug flags in matrix_init.