Update doc on displayhint in command -var-list-children

Update doc on displayhint in command -var-list-children

Hi,
When using command -var-list-children, "displayhint" appears in the result of
each child, shown as the following output.

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

Current doc on command -var-list-children doesn't reflect this.  This
patch is to fix it.

gdb/doc:

2013-11-23  Yao Qi  <yao@codesourcery.com>

	* gdb.texinfo (GDB/MI Variable Objects): Add the description
	of "displayhint" to the table about child results.