[common-rst] Modifications in conventions.rst
[common-rst] Modifications in conventions.rst

In this patch I added a 'caution' in conventions list
and also modified the explanation for note

Reason: The 'caution' is used in the
file admin-guide-cloud/source/blockstorage-driver-filter-weighing.rst

Change-Id: Ibb1aafad9d7668c666638622d9283a94ad180558

openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals