pgsql: doc: Add note about how to use pg_overexplain.

doc: Add note about how to use pg_overexplain.

This commit adds a note to the pg_overexplain page that describes
how to use it (LOAD, session_preload_libraries, or
shared_preload_libraries). The new text is mostly lifted from the
auto_explain page. We should probably consider centralizing this
information in the future.

While at it, add a missing "module" to the opening sentence.

Reviewed-by: "David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>
Reviewed-by: Robert Treat <rob(at)xzilla(dot)net>
Reviewed-by: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Discussion: https://postgr.es/m/aHVWKM8l8kLlZzgv%40nathan
Backpatch-through: 18

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/26cc96d4521acb598ddcd886bd64653452c7e887

Modified Files
--------------
doc/src/sgml/pgoverexplain.sgml | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)