qapi/block-core: Improve x-blockdev-change documentation

The description of feature @unstable is three paragraphs.  The second
and third became part of the description by accident in commit
9fb49daabfb (qapi: Mark unstable QMP parts with feature 'unstable').

The second paragraph describes a defect in terms of the
implementation.  Fine, but doesn't belong into user-facing
documentation.  Turn it into a TODO section.

Rewrite everything else for clarity and completeness.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250311131715.1296101-1-armbru@redhat.com>
Acked-by: Alberto Garcia <berto@igalia.com>

qapi/block-core.json

@@ -5913,35 +5913,31 @@
 ##
 # @x-blockdev-change:
 #
-# Dynamically reconfigure the block driver state graph.  It can be
-# used to add, remove, insert or replace a graph node.  Currently only
-# the Quorum driver implements this feature to add or remove its
-# child.  This is useful to fix a broken quorum child.
+# Dynamically reconfigure the block driver state graph.
 #
-# If @node is specified, it will be inserted under @parent.  @child
-# may not be specified in this case.  If both @parent and @child are
-# specified but @node is not, @child will be detached from @parent.
+# Currently only supports adding and deleting quorum children.  A
+# child will be added at the end of the list of children.  Its
+# contents *must* be consistent with the other childrens' contents.
+# Deleting a child that is not last in the list of children is
+# problematic, because it "renumbers" the children following it.
 #
 # @parent: the id or name of the parent node.
 #
-# @child: the name of a child under the given parent node.
+# @child: the name of a child to be deleted.  Mutually exclusive with
+#     @node.
 #
-# @node: the name of the node that will be added.
+# @node: the name of the node to be added.  Mutually exclusive with
+#     @child.
 #
 # Features:
 #
-# @unstable: This command is experimental, and its API is not stable.
-#     It does not support all kinds of operations, all kinds of
-#     children, nor all block drivers.
+# @unstable: This command is experimental.
 #
-#     FIXME Removing children from a quorum node means introducing
+# TODO: Removing children from a quorum node means introducing
 #     gaps in the child indices.  This cannot be represented in the
 #     'children' list of BlockdevOptionsQuorum, as returned by
 #     .bdrv_refresh_filename().
 #
-#     Warning: The data in a new quorum child MUST be consistent with
-#     that of the rest of the array.
-#
 # Since: 2.7
 #
 # .. qmp-example::