pgsql: Further cleanup related to statistics import support in postgres

Further cleanup related to statistics import support in postgres_fdw.

* Reorder the conditions in an if-else block for efficiency.
* Reorder struct definitions for readability.
* Reorder arguments for some functions for consistency.
* Move variables to the required scope.
* Rename a variable to match an input argument.
* Add/Tweak some comments/docs for clarity.

Author: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAPmGK14otpa7XZyBO5GxDABUK9OPWFLsAwoQP3ugNdXq%3DK4J1g%40mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5c52858ecdc052b2a9fc2a7f30e338ec6777c4ae

Modified Files
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contrib/postgres_fdw/postgres_fdw.c | 136 +++++++++++++++++++-----------------
doc/src/sgml/fdwhandler.sgml | 2 +-
src/backend/commands/analyze.c | 5 ++
3 files changed, 76 insertions(+), 67 deletions(-)