Improve doc for `\UseExerciseTemplate`

Currently the doc for \UseExerciseTemplate is

Lines 1545 to 1548 in 99d8b80

    
           \command{UseExerciseTemplate}[\marg{type}\marg{name}] 
        
             Retrieve template \meta{name} of type \meta{type}.  This can be useful if 
        
             you want to define a template which just adds some code to an existing 
        
             template (an automated \cs*{label}, say).

The allowed value of <type> seems to be one of begin and end, which is not documented.

It's actually any type declared through \xsim_new_template_type:n. Four types exist per default: begin, end, table and heading. I'll add these to the manual.

resolved in dc1222c

many thanks

	\command{UseExerciseTemplate}[\marg{type}\marg{name}]
	Retrieve template \meta{name} of type \meta{type}. This can be useful if
	you want to define a template which just adds some code to an existing
	template (an automated \cs*{label}, say).