pgAdmin III commit: Update the pgAgent installation docs now it has EXT

Update the pgAgent installation docs now it has EXTENSION support.

Branch
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REL-1_18_0_PATCHES

Details
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http://git.postgresql.org/gitweb?p=pgadmin3.git;a=commitdiff;h=f3e5a17e2cecb2af6e52d625ebba7f61500a792a
Author: David Wheeler 

Modified Files
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docs/en_US/pgagent-install.rst |   17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)