In some examples, use of `keysym` and `keycombo` seem wrong

[lo48576]

1. About use of keysym for key combinations
2. About use of keycombo without action attributes

Use of keysym for key combinations

keysym's description is below:

The keysym identifies the symbolic name of a key on a computer keyboard. This is distinct from any scan code that it may generate (keycode), or any text (keycap) that might be printed on the key.

However, some pages in the tdg.docbook.org reference, keysym is used to represent key combinations:
https://github.com/docbook/defguide/search?utf8=%E2%9C%93&q=keysym&type=

Here is one of such examples:

defguide/en/refpages/examples/guimenu.1.xml

Line 5 in daccd4d

<keycombo><keysym>C-c</keysym><keysym>C-x</keysym></keycombo>

Is this usage (<keysym>C-x</keysym>) valid?
I think this is invalid use (because keysym "identifies the symbolic name of a key on a computer keyboard"), and it should be written as <keycombo action="simul"><keysym>C</keysym><keycap>x</keycap></keycombo> (for example) or something like that.

If <keysym>C-x</keysym> is ok, the description of keysym element should be changed.

Use of keycombo without action attributes

In the same parts of examples:

defguide/en/refpages/examples/guimenu.1.xml

Line 5 in daccd4d

<keycombo><keysym>C-c</keysym><keysym>C-x</keysym></keycombo>

<keycombo><keysym>C-c</keysym><keysym>C-x</keysym></keycombo> is used and this seems to represent "sequential" combination.
But in the reference, action attribute is decribed as below:

If keycombo contains more than one element, simul is the default, otherwise there is no default.

The keycombo contains two keysyms, so this keycombo should have action="seq".
I think this should be fixed, whether the keysym use here is correct or nested keycombo should be used.