Documentation/capabilities-guide: remove --cap-retain/remove mentions

[s-urbaniak]

Partially fixes #3179 (the documentation part)

[alban]

https://github.com/coreos/rkt/blob/master/Documentation/subcommands/prepare.md needs fixing too.

And the rkt prepare command still use --cap-retain / --cap-remove.

[s-urbaniak]

@alban thanks, good catch!

[s-urbaniak]

self-merging

[alban]

@s-urbaniak But the old flags are still in some places in the documentation, and in the code:

$ git grep -- --cap-remove

$ git grep -- --cap-retain

[s-urbaniak]

@alban you are right, somehow my grep commands missed that, I'll open another issue to fix the remaining ones.