pgsql: Doc: declutter CREATE TABLE synopsis.

Doc: declutter CREATE TABLE synopsis.

Factor out the "persistence mode" and storage/compression parts
of the syntax synopsis to reduce line lengths and increase
readability.  Also add an introductory para about the persistence
modes so that the Description section still lines up with the
synopsis.

Author: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CAKFQuwYfMV-2SdrP-umr5SVNSqTn378BUvHsebetp5=DhT494w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a6d26e0fb29b8b644a495a6c2d5ed5c50b41addb

Modified Files
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doc/src/sgml/ref/create_table.sgml    | 38 ++++++++++++++++++++++++++---------
doc/src/sgml/ref/create_table_as.sgml |  8 ++++++--
2 files changed, 35 insertions(+), 11 deletions(-)