Merge #719

719: Add basic docs for timer and semaphore syscalls r=stlankes a=nathanwhyte

Helping with issue #353

Added _(very)_ basic documentation to syscalls in [`timer.rs`](https://github.com/hermitcore/libhermit-rs/blob/master/src/syscalls/timer.rs) and [`semaphore.rs`](https://github.com/hermitcore/libhermit-rs/blob/master/src/syscalls/semaphore.rs

)

I used Linux man pages as a reference when writing these.

Co-authored-by: Nathan Whyte <nathanwhyte35@gmail.com>