Document the signals pallet in the user-facing docs

docs/evolutions/index.md

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-title: Evolutions
-layout: default
-nav_order: 5
-has_children: true
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docs/protocol_changes/index.md

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+---
+title: Protocol Changes
+layout: default
+nav_order: 5
+---
+
+The protocol has no central authority nor organization nor actors (such as
+liquidity providers/validators) who can compel new protocol rules. The Serai
+protocol is as-written with all granted functionality and declared rules
+present.
+
+Validators are explicitly granted the ability to signal for two things to occur:
+
+1) Halt another validator set.
+
+This will presumably occur if another validator set turns malicious and is the
+expected incident response in order to apply an economic penalty of ideally
+greater value than damage wrecked. Halting a validator set prevents further
+publication of `Batch`s, preventing improper actions on the Serai blockchain,
+and preventing validators from unstaking (as unstaking only occurs once future
+validator sets have accepted responsibility, and accepting responsibility
+requires `Batch` publication). This effectively burns the malicious validators'
+stake.
+
+2) Retire the protocol.
+
+A supermajority of validators may favor a signal (an opaque 32-byte ID). A
+common signal gaining sufficient favor will cause the protocol to stop producing
+blocks in two weeks.
+
+Nodes will presumably, as individual entities, hard fork to new consensus rules.
+These rules presumably will remove the rule to stop producing blocks in two
+weeks, they may declare new validators, and they may declare new functionality
+entirely.
+
+While nodes individually hard fork, across every hard fork the state of the
+various `sriXYZ` coins (such as `sriBTC`, `sriETH`, `sriDAI`, and `sriXMR`)
+remains intact (unless the new rules modify such state). These coins can still
+be burned with instructions (unless the new rules prevent that) and if a
+validator set doesn't send `XYZ` as expected, they can be halted (effectively
+burning their `SRI` stake). Accordingly, every node decides if and how to future
+participate, with the abilities and powers they declare themselves to have.