Improve docs about timestamp type of pipeline transform

[sunng87]

https://docs.greptime.com/user-guide/logs/pipeline-config#the-type-field

epoch: Timestamp type, which will be converted to GreptimeDB timestamp(n) type. The value of n depends on the precision of the epoch. When the precision is s, n is 0; when the precision is ms, n is 3; when the precision is us, n is 6; when the precision is ns, n is 9.

We didn't mention how to specify precision for timestamp type, after checking code, it's written like this:

• timestamp,nanosecond
• timestamp,nano
• timestamp,ns

we will need to specify this in docs, an example is welcomed.