Prune doc references to unsupported PG versions

[jcflack]

17b7d7c (PR #450) dropped conditional code for PostgreSQL before 9.5, the oldest version to be supported by PL/Java 1.6.x. References to older PostgreSQL versions in the docs, however, were not pruned then. Remove those now.

The docs also mentioned some paths and other details that changed with the new build system in 1.6; bring those up to date. Also, link to the page on pljava.policy from a couple more places that only touched on policy generally. These changes are not strictly about pre-9.5 PostgreSQL versions, but are details that were similarly overlooked in the PL/Java 1.5 to 1.6 transition.