Cleanup: remove and/or fix some confusing comments

The comments for SettingDescType; it is a byte, so not 4 bytes and since it is not a flag there are about 250 other possibilities left instead of 9.
SettingGuiFlag is uint16 so has 2 bytes allocated.
SettingDescGlobVarList and related comments imply that global vars cannot be used elsewhere, but they are used for settings just fine. Even then the type is not used anywhere else but the definition of the table.

src/table/settings.h.preamble

@@ -23,9 +23,7 @@ static size_t ConvertLandscape(const char *value);
  * The macros can be grouped depending on where the config variable is
  * stored:
  * 1. SDTG_something
- *    These are for global variables, so this is the one you will use
- *    for a #SettingDescGlobVarList section. Here 'var' refers to a
- *    global variable.
+ *    These are for global variables. Here 'var' refers to a global variable.
  * 2. SDTC_something
  *    These are for client-only variables. Here the 'var' refers to an
  *    entry inside _settings_client.