Add note about debug levels for short help stdout

[przepompownia]

• Geeqie 2.6+git20250616-0c75926a GTK3

Now geeqie --help shows

--debug=[level]                            turn on debug output

without the range and short explanation of level.

It would be convenient not to have to look through the documentation, code or online resources (like this) to find that information.

[caclark]

I will add some documentation.

debug=<n> is in the range of 1 to 4.

Level 1 has less information that levels 2 to 4.

Level <n> will output all lower numbered levels.

Unfortunately there is no logic in how the debug statements have been used in the code. It is necessary to use the source code in combination with the debug output.

[przepompownia]

I think that displaying the range 1-4 and that 4 is the max verbosity should be enough at that output.