doc: blobs: Document startup notification

Due to the (potentially) hard to understand effects of blobs, it seems
prudent to make their presence more noticeable.

Hopefully, this will make it easier to identify issues (such as those
reported on GitHub) that have been observed on devices running a tainted
build.

Signed-off-by: Reto Schneider <[email protected]>

Author: rettichschnidi

doc/contribute/bin_blobs.rst

@@ -92,6 +92,7 @@ Tainting will be communicated to the user in the following manners:
 - The ``west spdx`` command will include the tainted status in its output
 - The kernel's default fatal error handler will also explicitly print out the
   kernel's tainted status
+- The boot banner prints the kernel's tainted status
 
 .. _bin-blobs-types: