Improve document about the setting to preload pg_bigm shared library to server.

Improve document about the setting to preload pg_bigm shared library to server.

This commit adds the description explaining that, in PostgreSQL 9.4 or later,
session_preload_libraries parameter instead of shared_preload_libraries can
be used to specify the pg_bigm shared library to preload to the server, into
the document.

Backpatch to 1.1 where 9.4 is supported.

Reviewed by Amit Langote