Clarify signal documentation

[DL6ER]

After a report on Discourse, investigations have revealed that SIGRTMIN are defined differently in Debian (glibc-based) and Alpine (musl-based). Hence, sending signals relative to SIGRTMIN is going to be an issue on Debian as we build the pihole-FTL binaries on Alpine. Change the documentation to use fixed signals (instead of relative ones) to fix this. Signals relative to the constant SIGRTMIN have been added as there is a certain number ambiguity under glibc (see man 7 signals). However, this ambiguity does not exist in musl-based binaries.

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This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/signal-documentation-for-v6/76424/9

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