nqp::setobjsc and nqp::scsetobj are not documented

[moritz]

The two ops mentioned in the title are missing from docs/ops.markdown.

[Kaiepi]

Ditto the other serialization context ops

[MasterDuke17]

See also https://github.com/perl6/nqp/issues/293

[coke]

Literally the opcodes are there, but they were added as placeholders with no docs, so still needs resolution

[coke]

• [ ] getobjsc
• [ ] popcompsc
• [ ] pushcompsc
• [ ] scdisclaim
• [ ] scgetobj
• [ ] scgetobjidx
• [ ] scobjcount
• [ ] scsetobj
• [ ] scwbdisable
• [ ] scwbenable
• [ ] setobjsc