Clarify parameters about cpu/mem for the cluster to avoid confusing

[yecol]

Reported by our users, e.g., vineyard_shared_mem is not clear, users may be confused about whether it means the shared_mem on a node or the total amount on the cluster. Suggestion: v6d_mem_each_node maybe? (Similar to other parameters in the session configurations.)

[siyuan0322]

I have some clarifications here: https://graphscope.io/docs/frequently_asked_questions.html

[sighingnow]

We could add a "Parameter Tunning" or "Performance Tunning" section once after the demo section in README.md.