Add links to with_only_columns to Select.column, append_column

Add links to with_only_columns to Select.column, append_column

Provide a brief example for these two methods
indicating that typically a table-bound (or other selectable)
column is appended here, then link to with_only_columns
documentation which has in-depth guidelines already including
that one should not append columns from the current select to itself.

Change-Id: I0742405a7f3c41450d337b9c633519d9cc101dfb
Fixes: #3987