Add better explanations of examples in the C API section of the docs

[matea16]

A user has provided a valuable feedback regarding the C API section of the docs:

It would be useful to have better explanation or examples about the main library Query modules C API since the other languages are just a wrapper and their methods have pretty similar naming. Also I think would be useful to have some methods for filtering, like getting nodes based on labels instead getting the whole graph and checking in a loop if a vertex has a label.

Action points:

• Expand the explanation of the existing examples
• Update docs with the section on filtering methods