[DOCS] Improving the description of several parameters

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### Which parts of the documentation do you think need improvement?

The following configurations are a bit confusing

> rss.coordinator.shuffle.nodes.max
is assignment per shuffle partition or per shuffle writer? why can there be multiple assignments in the first place?

> rss.coordinator.remote.storage.select.strategy
what does `IO_BALANCE` and `APP_BALANCE` even mean?

> rss.server.buffer.capacity
is this a shared global buffer or per app buffer or per connection buffer? how to tune this? what to look for?

> rss.server.read.buffer.capacity
is this global or per shuffle reader? how to tune it? what to look for?

> rss.server.single.buffer.flush.enabled
what is a single buffer?

### Affects Version(s)

master

### Improving the documentation

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

Start with the configuration definitions and usages for the five properties listed in issue #955. Trace how each setting is applied, then update the relevant documentation to explain assignment scope, strategy meanings, buffer scope and tuning guidance, and single-buffer terminology. Verify that all five questions are answered consistently.

Written by the indexing model from the issue text.