Document the proper use of click handlers.

[Poikilos]

Some use extra_bindings.

Example:

        self.sheet = Sheet(
            self,
            show_row_index=False,
        )
        self.sheet.headers(self.sheet_headers)

        # Key bindings only seem to work in root, not the sheet:
        self.root.bind("<F3>", lambda event: self.find_next())

        # Allow selection and editing
        self.sheet.enable_bindings((
            "single_select",     # Allow single cell selection
            # "row_select",        # Allow row selection
            # "column_select",     # Allow column selection
            "arrowkeys",         # Allow navigation using arrow keys
            # "edit_cell",         # Enable cell editing
            "double_click",      # Enable double-click functionality
            # "rc_select",         # Allow right-click selection
            # "rc_insert_column",  # Enable right-click column insert
            # "rc_delete_column",  # Enable right-click column delete
            # "rc_insert_row",     # Enable right-click row insert
            # "rc_delete_row",     # Enable right-click row delete
            "copy",              # Enable copy to clipboard
            "cut",               # Enable cut to clipboard
            # "paste"              # Enable paste from clipboard
        ))
        self.sheet.extra_bindings("cell_select", self.on_cell_click)
        self.sheet.bind("<Double-1>", self.on_cell_double_click)