pgsql: Document that Slony can do upgrades easier _because_ it supports

Document that Slony can do upgrades easier _because_ it supports
replication between different Postgres major versions.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=e8cdc238cfa3fda4185fe6a9826dbdf0a107f789

Modified Files
--------------
doc/src/sgml/runtime.sgml | 2 ++
1 files changed, 2 insertions(+), 0 deletions(-)