legal: remove doc license ambiguity

legal: remove doc license ambiguity

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 ed0653b493a3f919a3abc37a0aa9b5aa29ae0b0e

This license declaration omitted a version number.

Some time later ef7418421b3748c712019c8aedd02b8005c1e1ea 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>
(cherry picked from commit e294dd59100d962753f4dee8f7156be5a7fba875)