Doc 'dynamic' for command -var-list-children

Hi,
I find "dynamic=1" appear in the result of each child of the output of
-var-list-children,

  -var-list-children  ss1
  ^done,numchild="2",children=[child={name="ss1.a",exp="a",numchild="0",type="struct s",thread-id="1",dynamic="1"},child={name="ss1.b",exp="b",numchild="0",type="struct s",thread-id="1",dynamic="1"}],has_more="0"

but the doc doesn't mention this.  This patch is to copy the description
of "dynamic=1" here.

gdb/doc:

2013-11-21  Yao Qi  <yao@codesourcery.com>

	* gdb.texinfo (GDB/MI Variable Objects): Add attribute 'dynamic'
	for the output of command -var-list-children.