Documentation of rist_destroy()

Hi,

The function rist_destroy() is documented as "Destroy RIST sender/instance" here. In librist.h, the comments also mention "Destroy RIST sender".

However, the source code demonstrates that it is used to equally destroy sender and receiver contexts. This could misguide some developers who (if very naive...) could think that a receiver is magically destroyed without explicit call, leading to resource leaks.

Additionally, it is a bit confusing that most deallocation functions are named as xxx_free(), except rist_peer_destroy() and rist_destroy(). Just a matter of naming consistency.