pgsql: Clarify that --system reindexes system catalogs *only*

Clarify that --system reindexes system catalogs *only*

Make this more clear both in the help message and docs.

Reviewed-By: Michael Paquier
Backpatch-through: 9.6
Discussion: /messages/by-id/CABUevEw6Je0WUFTLhPKOk4+BoBuDrE-fKw3N4ckqgDBMFu4paA@mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/162cb0834a9a06ac076d613729447ccf5c2ee21b

Modified Files
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doc/src/sgml/ref/reindexdb.sgml | 2 +-
src/bin/scripts/reindexdb.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)