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.g.johnston@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: http://postgr.es/m/aHVWKM8l8kLlZzgv%40nathan
Backpatch-through: 18