The license applicable to the files under doc/ was originally declared (in the
top-level file COPYING) to be "Creative Commons Attribution-ShareAlike (CC
BY-SA)" by ed0653b4

This license declaration omitted a version number.

Some time later ef741842 was merged, mentioning
CC-BY-SA-1.0 as one of the Ceph source code licenses. Although the purpose of that
commit was only to summarize the canonical license information from COPYING, it
unintentionally became the only place in the source code where the doc license
version was specified.

In March 2018, the Debian project warned that CC-BY-SA-1.0 does not meet its
criteria for inclusion in the "free" section of the Debian distribution. (For
that, at least version 3.0 must be used.)

This new commit removes the doc license ambiguity by setting it to CC-BY-SA-3.0
in all places where the license is mentioned.

The exact spelling of the license name is taken from https://spdx.org/licenses/

Fixes: http://tracker.ceph.com/issues/23336


Signed-off-by: Nathan Cutler <ncutler@suse.com>