doc: update intermediate certificate instructions

Document how to properly create root and intermediate certificates using
v3_ca extensions and where to place intermediate certificates so they
are properly transferred to the remote side with the leaf certificate to
link to the remote root certificate.  This corrects docs that used to
say that intermediate certificates must be stored with the root
certificate.

Also add instructions on how to create root, intermediate, and leaf
certificates.

Discussion: https://postgr.es/m/20180116002238.GC12724@momjian.us

Reviewed-by: Michael Paquier

Backpatch-through: 9.3