config: Make capabilities and noNewPrivileges Linux-only (again)
config: Make capabilities and noNewPrivileges Linux-only (again)

Roll back the genericization from 718f9f3f (minor narrative cleanup
regarding config compatibility, 2017-01-30, #673).  Lifting the
restriction there seems to have been motivated by "Solaris supports
capabilities", but that was before the split into a capabilities
object which happened in eb114f05 (Add ambient and bounding capability
support, 2017-02-02, #675).  It's not clear if Solaris supports
ambient caps, or what Solaris API noNewPrivileges were punting to [1].
And John Howard has recently confirmed that Windows does not support
capabilities and is unlikely to do so in the future [2].  He also
confirmed that Windows does not support rlimits [3].  John's statement
didn't directly address noNewPrivileges, but we can always restore any
of these properties to the Solaris/Windows platforms if/when we get
docs about which API we're punting to on those platforms.

Also add some backticks, remove the hyphens in "OPTIONAL) - the",
standardize lines I touch to use "the process" [4], and use four-space
indents here to keep Pandoc happy (see 7795661 (runtime.md: Fix
sub-bullet indentation, 2016-06-08, #495).

[1]: https://github.com/opencontainers/runtime-spec/pull/673#discussion_r99353136
[2]: https://github.com/opencontainers/runtime-spec/pull/810#issuecomment-301594590
[3]: https://github.com/opencontainers/runtime-spec/pull/835#issuecomment-303455386
[4]: https://github.com/opencontainers/runtime-spec/pull/809#discussion_r116297660

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

hqhq/specs,laijs/specs,wking/opencontainer-specs,opencontainers/specs,hqhq/runtime-spec,wking/opencontainer-runtime-spec,opencontainers/runtime-spec