pgsql: 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 <laurenz.albe@cybertec.at>
Reported-by: Lauren Fliksteen <dancernerd32@gmail.com>
Reviewed-by: Rajakavitha Kodhandapani <krajakavitha@gmail.com>
Discussion: https://postgr.es/m/165290238488.670.7500177735573254738@wrigleys.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a5084a10fe60412036899a79b3aee76ba31a3579

Modified Files
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doc/src/sgml/ref/create_index.sgml | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)