setpriv: document accepted formats for naming caps

setpriv: document accepted formats for naming caps

Since commit fbd15c4d4 (setpriv: support setting unnamed capabilities,
2017-07-17), it is possible to name capabilities not only by name, but
also by their index. While using the human-readable name is usually
recommended, using the index may be required in case new capabilities
have been introduced to the kernel for which we have no name yet. The
newly introduce format format is not documented inside of setpriv(1),
though, which is being fixed in this commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>