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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b3d33bf598dd64c95f48111081b6672ec0dc380e

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