doc: Explain 'invalid' index state for CREATE INDEX CONCURRENTLY

doc: Explain 'invalid' index state for CREATE INDEX CONCURRENTLY

It wasn't previously mentioned that the index is created as invalid,
which is confusing to new users.

Backpatch to 14 (only because of a conflict in 13).

Author: Laurenz Albe <[email protected]>
Reported-by: Lauren Fliksteen <[email protected]>
Reviewed-by: Rajakavitha Kodhandapani <[email protected]>
Discussion: https://postgr.es/m/165290238488.670.7500177735573254738@wrigleys.postgresql.org