[BugBash][CLI]descriptions of hub commands are not clear enough

### Describe the bug
[BugBash][CLI]descriptions of hub commands are not clear enough

### To Reproduce

view the help of comand `az webpubsub hub create -h`. better to have:
- `--allow-anonymous`: include the default value in description. And the input `false`/`true` is not consistent with the actual output `allow`/`deny`. Better to explain their relationship.
- `--event-handler`: better to clearly explain that `user-event-pattern` is a string. If included mutliple times the last will be used while others ignored. But for `system-event`, it's a array, all inputs will be combined together as an array. Besides, it doesn't take a string seperated by comma or other chars.

### Exceptions (if any)

### Further technical details
- SDK and its version you use