nixos/treewide: remove boolean examples for options

They contain no useful information and increase the length of the
autogenerated options documentation.

See discussion in #18816.

nixos/modules/services/computing/boinc/client.nix

@@ -12,7 +12,6 @@ in
       enable = mkOption {
         type = types.bool;
         default = false;
-        example = true;
         description = ''
           Whether to enable the BOINC distributed computing client. If this
           option is set to true, the boinc_client daemon will be run as a
@@ -41,7 +40,6 @@ in
       allowRemoteGuiRpc = mkOption {
         type = types.bool;
         default = false;
-        example = true;
         description = ''
           If set to true, any remote host can connect to and control this BOINC
           client (subject to password authentication). If instead set to false,