Explicitly document the fact that “blacklist” doesn't prevent dependency loading and shouldn't be used for security

[Gro-Tsen]

The current description of the blacklist keyword in the modprobe.d(5) man page correctly indicates that “all of that particular module's internal aliases are to be ignored”, but the name of the keyword itself is likely to cause confusion and lead some users into believing, as I did, that this will prevent the module from being loaded or even just auto-loaded. This is particularly problematic in the case of a security vulnerability (I thought I was making myself safe from CVE-2026-31431 by adding blacklist algif_aead in my modprobe config: I was not).

I believe a warning in the man page would be useful to let users know that blacklist neither prevents explicit loading of the module, nor loading as a dependency, and should not be used as a security measure. Something like this:

Warning: despite what the keyword may appear to suggest, blacklist only suppresses alias-based automatic loading. In particular, the designated module can still be loaded as a dependency of another module (or explicitly). Consequently, blacklist should generally not be used for security purposes: using install foobar /bin/false instead is typically safer (see install for more).

[I am submitting a separate issue, #440, requesting a friendlier keyword instead of install foobar /bin/false. If this is approved, the warning should, of course, point to that instead.]