pgsql: Document concurrent indexes waiting on each other

Document concurrent indexes waiting on each other

Because regular CREATE INDEX commands are independent, and there's no
logical data dependency, it's not immediately obvious that transactions
held by concurrent index builds on one table will block the second phase
of concurrent index creation on an unrelated table, so document this
caveat.

Backpatch this all the way back. In branch master, mention that only
some indexes are involved.

Author: James Coleman <jtc331(at)gmail(dot)com>
Reviewed-by: David Johnston <david(dot)g(dot)johnston(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAaqYe994=PUrn8CJZ4UEo_S-FfRr_3ogERyhtdgHAb2WG_Ufg@mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3fe0e7c3fa27de80419de9ce66be2767d2ddae57

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