pgsql: Improve documentation of postgres -C option

Improve documentation of postgres -C option

Clarify help (s/return/print/), and explain that this option is for
use by other programs, not for user-facing use (it does not print
units).

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/c0a6f9c84bedc745af2a3bf1dee5e2a4e79b46af

Modified Files
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doc/src/sgml/ref/postgres-ref.sgml |    9 ++++++++-
src/backend/main/main.c            |    2 +-
2 files changed, 9 insertions(+), 2 deletions(-)