[Core] Better document resource dict and its relationship to num_cpu, etc.

[scottsun94]

Description

@pcmoritz mentioned that num_cpus=n is a ray.remote argument which is a convenient short hand for resources={"CPU": n}, and whenever we expect a resource dict, the only way to really specify it is via {"CPU": n}.

We should better document this:

• having a section in the docs on what exactly a resource dict is and how it relates to num_cpu
• link to this section from wherever we expect resource dicts.

Link

No response