Upgrade/update instructions for 5.1; relnotes descriptions

This commit covers all the docs about upgrade and update
as well as refinements to the what-is, new-features, and
other-enhancements sections.

Includes change to use lrzuntar to unpack the 5.1 tarball
with a warning that the Fuel Master node requires 2GB RAM for
this to work, with a link to the hardware requirements for
the master

Includes corrections to the Release Notes Upgrade and Update sections
and to this information in the User Guide.

user-guide/cli.rst -- info from https://review.openstack.org/120428
about the update option.

Ref-Arch now includes 8000-fuel-upgrade that describes how
upgrade works

Release Notes are restructured to include:
- "Known Issues fixed in 5.1 and 5.0.2" is added in 117672
- "HA issues fixed in 5.1" is added in 117338

Change-Id: Ie2054b31ce216942fd64b5bf274d8efa7d300bc4
Closes-Bug: 1360372
Closes-Bug: 1367199