### What this PR does / why we need it?
Update Memcache local service config example: increase default world
size to 256 and update the description for better clarity.
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
- vLLM version: v0.16.0
- vLLM main:
4034c3d32e
Signed-off-by: Pz1116 <zpbzpb123123@gmail.com>