Document PIHOLE_SELINUX environment variable

[TomPi-22]

What does this PR aim to accomplish?:

Documents the PIHOLE_SELINUX environment variable, which is currently undocumented anywhere on docs.pi-hole.net although it is present in the current basic-install.sh. Provides a reference for users who encounter the installer's SELinux Enforcing exit.

Fixes #316

How does this PR accomplish the above?:

Added a new "SELinux" section to docs/main/update.md. It explains that pihole -up will exit if SELinux is detected in Enforcing mode, and documents how setting PIHOLE_SELINUX=true before running the update skips that check.

Link documentation PRs if any are needed to support this PR:

N/A

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