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

Documentation/process/maintainer-tip.rst

@@ -421,6 +421,9 @@ allowing themselves a breath. Please respect that.
 The release candidate -rc1 is the starting point for new patches to be
 applied which are targeted for the next merge window.
 
+So called _urgent_ branches will be merged into mainline during the
+stabilization phase of each release.
+
 
 Git
 ^^^