## What changes were proposed in this pull request?
This PR targets to document `-I` option from Spark 2.4.x (previously `-i` option until Spark 2.3.x).
After we upgraded Scala to 2.11.12, `-i` option (`:load`) was replaced to `-I`(SI-7898). Existing `-i` became `:paste` which does not respect Spark's implicit import (for instance `toDF`, symbol as column, etc.). Therefore, `-i` option does not correctly from Spark 2.4.x and it's not documented.
I checked other Scala REPL options but looks not applicable or working from quick tests. This PR only targets to document `-I` for now.
## How was this patch tested?
Manually tested.
**Mac:**
```bash
$ ./bin/spark-shell --help
Usage: ./bin/spark-shell [options]
Scala REPL options:
-I <file> preload <file>, enforcing line-by-line interpretation
Options:
--master MASTER_URL spark://host:port, mesos://host:port, yarn,
k8s://https://host:port, or local (Default: local[*]).
--deploy-mode DEPLOY_MODE Whether to launch the driver program locally ("client") or
on one of the worker machines inside the cluster ("cluster")
(Default: client).
...
```
**Windows:**
```cmd
C:\...\spark>.\bin\spark-shell --help
Usage: .\bin\spark-shell.cmd [options]
Scala REPL options:
-I <file> preload <file>, enforcing line-by-line interpretation
Options:
--master MASTER_URL spark://host:port, mesos://host:port, yarn,
k8s://https://host:port, or local (Default: local[*]).
--deploy-mode DEPLOY_MODE Whether to launch the driver program locally ("client") or
on one of the worker machines inside the cluster ("cluster")
(Default: client).
...
```
Closes#22919 from HyukjinKwon/SPARK-25906.
Authored-by: hyukjinkwon <gurwls223@apache.org>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>
cc38abc27a