docs/user: Document the %d placeholder and suspend=n QEMU_GDB features

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20250117001542.8290-8-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250207153112.3939799-17-alex.bennee@linaro.org>

docs/user/main.rst

@@ -54,7 +54,7 @@ Command line options
 
 ::
 
-   qemu-i386 [-h] [-d] [-L path] [-s size] [-cpu model] [-g port] [-B offset] [-R size] program [arguments...]
+   qemu-i386 [-h] [-d] [-L path] [-s size] [-cpu model] [-g endpoint] [-B offset] [-R size] program [arguments...]
 
 ``-h``
    Print the help
@@ -91,8 +91,18 @@ Debug options:
    Activate logging of the specified items (use '-d help' for a list of
    log items)
 
-``-g port``
-   Wait gdb connection to port
+``-g endpoint``
+   Wait gdb connection to a port (e.g., ``1234``) or a unix socket (e.g.,
+   ``/tmp/qemu.sock``).
+
+   If a unix socket path contains single ``%d`` placeholder (e.g.,
+   ``/tmp/qemu-%d.sock``), it is replaced by the emulator PID, which is useful
+   when passing this option via the ``QEMU_GDB`` environment variable to a
+   multi-process application.
+
+   If the endpoint address is followed by ``,suspend=n`` (e.g.,
+   ``1234,suspend=n``), then the emulated program starts without waiting for a
+   connection, which can be established at any later point in time.
 
 ``-one-insn-per-tb``
    Run the emulation with one guest instruction per translation block.