qga/qapi-schema: Add a proper introduction

Contents adapted from qapi/qapi-schema.json.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250404121413.1743790-12-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

qga/qapi-schema.json

@@ -2,10 +2,24 @@
 # vim: filetype=python
 
 ##
-# = QEMU guest agent protocol commands and structs
+# This manual describes the commands supported by the QEMU Guest
+# Agent Protocol.
 #
-# For a concise listing of all commands, events, and types in the QEMU
-# guest agent, please consult the `qapi-qga-index`.
+# For locating  a particular item, please see the `qapi-qga-index`.
+#
+# The following notation is used in examples:
+#
+# .. qmp-example::
+#
+#   -> ... text sent by client (commands) ...
+#   <- ... text sent by server (command responses and events) ...
+#
+# Example text is formatted for readability.  However, in real
+# protocol usage, its commonly emitted as a single line.
+#
+# Please refer to the
+# :doc:`QEMU Machine Protocol Specification </interop/qmp-spec>`
+# for the general format of commands, responses, and events.
 ##
 
 { 'pragma': { 'doc-required': true } }