pgsql: Add database comments to template0 and postgres databases, and i

Add database comments to template0 and postgres databases, and improve
the comments on the template1 database. No catalog version bump because
they are just comments.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ddd6ff289f2512f881493b7793853a96955459ff

Modified Files
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src/bin/initdb/initdb.c | 3 +++
src/include/catalog/pg_database.h | 2 +-
2 files changed, 4 insertions(+), 1 deletions(-)