write the step debugging guide page

docs/user-guide/step-debugging.md

@@ -2,8 +2,91 @@
 title: Step Debugging
 ---
 
-Step over, into, and out of your code with ease, following execution across files, closures, and includes.
+When a [breakpoint](./breakpoints.md) fires, your script freezes exactly where it
+is and the debugger hands control to your editor. Nothing else happens until you
+say so — PHP is sitting inside the request, holding its stack, its variables and
+its open connections, waiting.
 
-:::note
-This page is a work in progress — full documentation is coming soon.
-:::
+From there you have four ways to move, and two ways to finish.
+
+## Moving through the code
+
+| Action | What happens |
+| --- | --- |
+| Step into | Runs the next statement. If it calls a function, enter the function and stop on its first line. |
+| Step over | Runs the next statement, including any calls it makes, and stops on the line after without entering them. |
+| Step out | Runs the rest of the current function and stops just after it returns. |
+| Run | Carries on until the next breakpoint, or to the end of the request. |
+
+**Step into** is the one that follows your code down. It descends into your
+functions, methods and closures, and into files pulled in by `include` or
+`require`. Bear in mind that it descends into *everything* your code calls — step
+into a line that starts with a framework call and you can find yourself several
+layers deep in code you did not write.
+
+**Step over** is what you want on the lines you already trust. The call still
+happens, and its breakpoints still fire — stepping over a function does not disable
+what is inside it. It only means you do not want to watch.
+
+**Step out** is the way back up when you have descended too far, or when you have
+seen what you needed to and want to get back to the caller. If you are already at
+the top level, there is nothing to return to, and it behaves like Run.
+
+**Run** — your editor may call it Continue or Resume — continues running the
+script. If nothing else stops it, the request completes normally and the session
+ends with it.
+
+## Run to cursor
+
+Most editors offer a "run to cursor" or "run to here": put the caret somewhere
+ahead and the script continues until it reaches that spot.
+
+There is no such command in the debugger. What your editor does is set a
+[temporary breakpoint](./breakpoints.md#temporary-breakpoints) on that line and
+then Run. The breakpoint fires once and switches itself off, which produces exactly
+the behaviour you asked for.
+
+Worth knowing for two reasons. The line still has to be one that executes, exactly
+as with any other breakpoint — aim at a comment or a closing brace and the script
+will run straight past it to the end. And it is subject to everything else in the
+way: any other breakpoint that fires first will stop you before you arrive.
+
+## Seeing return values
+
+By default, stepping out of a function drops you on the line after the call, and
+the value that came back is not shown anywhere — you have to step on and inspect
+whatever it was assigned to. If it was not assigned to anything, or the call is
+part of a longer chain, that value is awkward to get at.
+
+The debugger can add an extra stop on the way out of each function, showing what it
+is about to return. This is off unless your editor asks for it — it is negotiated
+when the session starts, so where you turn it on is an editor setting, usually
+along the lines of "break on return" or "show return values".
+
+The cost is one more stop per function you step out of, which is the reason it is
+not on by default. Turn it on while you are chasing a wrong value through a chain
+of calls, and off again afterwards.
+
+## Stopping and detaching
+
+Two ways to end a session, and the difference matters:
+
+| Action | Effect |
+| --- | --- |
+| Stop | The script is killed. It does not finish. |
+| Detach | The debugger lets go and the script runs to completion on its own. |
+
+**Stop** terminates the request where it stands. Nothing after the current point
+runs — no remaining output, no shutdown functions, no destructors you were counting
+on. Anything half-finished stays half-finished: a transaction that was never
+committed, a file that was written but not closed, a queue message taken but not
+acknowledged.
+
+**Detach** disconnects the debugger and leaves the script running. Breakpoints stop
+firing, the request finishes as it normally would, and your editor goes quiet. This
+is the one to use when you have seen what you came for on a request that ought to
+complete — a checkout that should reach the end, a job that should be marked done.
+
+Neither turns the debugger off. The next request starts a fresh session and
+connects again, so if you are stepping through something and want the noise to stop
+for a while, disconnect your editor rather than detaching from each request in turn.