docs: document capnrpc/src/lib.rs

[SteveLauC]

What does this PR do

Document most types/interfaces in capnp-rpc/src/lib.rs.

Why

I started working on #495, the first thing was to read the source code, then I found these are a lot of concepts that I am not aware of, and they do not have documents. Considering this is a Rust port is the C++ implementation, authors of this port should be already familiar with those concepts, this is understandable.

But I am completely new to them, so I took a look at the C++ repo, found the concept I wanted to understand, and copied the documents into the Rust code.

For stuff that already exists in the C++ impl, the documents are directly pasted, documents of other things are added based on my understanding.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.73%. Comparing base (ab342b3) to head (8c86956).
Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
+ Coverage   51.64%   51.73%   +0.09%     
==========================================
  Files          69       69              
  Lines       33735    33808      +73     
==========================================
+ Hits        17422    17492      +70     
- Misses      16313    16316       +3     

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[dwrensha]

Thanks!