Docs: Assert cell has group

Explicitly assign $_TBUF_ to `gate_other` and remove catch if a cell has no group.

docs/source/cell_word.rst

@@ -28,5 +28,3 @@ Simulation models for the RTL cells can be found in the file
    /cell/word_formal
    /cell/word_debug
    /cell/word_wire
-
-.. todo:: Add check for unexpected ``word_other`` cells

techlibs/common/cellhelp.py

@@ -88,12 +88,7 @@ for line in fileinput.input():
             simHelper.desc.pop(1)
 
         # check group
-        if not simHelper.group:
-            if short_filename == 'simcells.v':
-                simHelper.group = "gate_"
-            elif short_filename == 'simlib.v':
-                simHelper.group = "word_"
-            simHelper.group += "other"
+        assert simHelper.group, f"techlibs/common/{simHelper.source}: {simHelper.name} cell missing group"
 
         # dump
         print(simHelper)

techlibs/common/simcells.v

@@ -461,6 +461,7 @@ endmodule
 //  |---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|
 //-
 //-     $_TBUF_ (A, E, Y)
+//* group gate_other
 //-
 //- A tri-state buffer.
 //-