docs: tags for commenting mappings without "-default" suffix (#28205)

This actually won't cause "duplicate tag" errors if plugins have tags of
the same name, because E154 is only given for duplicate tags in the same
directory.

Without those tags, trying to use :h for these mappings jumps to other
places, because there are matches with higher score.

runtime/doc/various.txt

@@ -581,16 +581,16 @@ to look up the value of 'commentstring' corresponding to the cursor position.
 (This can be different from the buffer's 'commentstring' in case of
 |treesitter-language-injections|.)
 
-							*gc-default*
+							*gc* *gc-default*
 gc{motion}		Comment or uncomment lines covered by {motion}.
 
-							*gcc-default*
+							*gcc* *gcc-default*
 gcc			Comment or uncomment [count] lines starting at cursor.
 
-							*v_gc-default*
+							*v_gc* *v_gc-default*
 {Visual}gc		Comment or uncomment the selected line(s).
 
-							*o_gc-default*
+							*o_gc* *o_gc-default*
 gc			Text object for the largest contiguous block of
 			non-blank commented lines around the cursor (e.g.
 			`gcgc` uncomments a comment block; `dgc` deletes it).