useLiveIncrementalQuery: addition to docs

[barbalex]

Here: https://pglite.dev/docs/framework-hooks/react#useliveincrementalquery

Please document, that the column to key the diff algorithm on needs to be included in the list of selected columns or else this message will occur:

error: column "column_to_key_the_diff_algorithm_on" does not exist

I guess this means that it shouldn't/can't be used for some types of queries like SELECT count(*)....