Update multisearch README rebuild example
Update multisearch README rebuild example

The provided double-pipe concatenation example has a gotcha regarding handling of NULL values, which can result in an "unexpected" empty `content` value. 

I've swapped this for a use of `CONCAT_WS()` which is more forgiving/readable, and has been supported since Postgres 9.1 (released 10 years ago). A note has been added for those unfortunate enough to be working on very old databases. Maybe this isn't even necessary, but some might consider the concatenation more "proper".
Casecommons/pg_search,Casecommons/pg_search