Documentation/process: Explain when tip branches get merged into mainline

Explain when tip branches get merged into mainline.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/8a1fd8b7-9fe3-b2b5-406e-fa6f5e03e7c0@nerdbynature.de