write the debug protocol page

docs/reference/debug-protocol.md

@@ -2,8 +2,45 @@
 title: Debug Protocol
 ---
 
-PHP Debugger speaks the DBGp protocol, making it compatible with any IDE or tool that speaks DBGp.
+The debugger and your editor talk to each other over **DBGp**, a debugging protocol
+originally designed for PHP and now spoken by every PHP debugging client worth
+using.
+
+That is why there is no editor plugin to install for PHP Debugger specifically.
+Your editor already knows how to hold a DBGp conversation; the debugger connects to
+it and they understand one another. Anything that could already debug PHP can debug
+your code here, with no adapter in between.
+
+Mechanically it is a small thing: a TCP connection opened by the debugger, carrying
+XML messages in both directions. The editor asks — set a breakpoint here, step over,
+give me the value of `$order` — and the debugger answers. Everything the
+[User Guide](../user-guide/starting-the-debugger.md) describes is that conversation
+under a friendlier name.
+
+## The specification
+
+The protocol is documented in full at
+[xdebug.org/docs/dbgp](https://xdebug.org/docs/dbgp) — every command, its arguments
+and the shape of its response.
+
+You do not need any of it to use the debugger. It is worth reading if you are
+building a client, debugging a client, or trying to work out why your editor and the
+debugger disagree about something — for which the raw message log at
+[level 5](./settings.md#php_debuggerlog_level) is the other half of the answer.
+
+## Compatibility
+
+PHP Debugger currently implements DBGp version 1.0 as specified, with no
+deliberate differences. If your editor works with the protocol, it works here.
+
+:::info[This may not always be exactly true]
+
+Keeping the protocol unchanged is a deliberate choice for now, not a permanent
+commitment. Some improvements we would like to make are difficult to express within
+the specification as it stands, and we may eventually add to it or diverge from it
+where the benefit is worth the cost.
+
+Anything of that kind would be documented here, and compatibility with existing
+clients is something we intend to keep rather than break casually.
 
-:::note
-This page is a work in progress — full documentation is coming soon.
 :::