[SPARK-25906][SHELL] Documents '-I' option (from Scala REPL) in spark-shell

## 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 apache#22919 from HyukjinKwon/SPARK-25906.

Authored-by: hyukjinkwon <[email protected]>
Signed-off-by: hyukjinkwon <[email protected]>
(cherry picked from commit cc38abc)
Signed-off-by: hyukjinkwon <[email protected]>

Author: HyukjinKwon

bin/spark-shell

@@ -32,7 +32,10 @@ if [ -z "${SPARK_HOME}" ]; then
   source "$(dirname "$0")"/find-spark-home
 fi
 
-export _SPARK_CMD_USAGE="Usage: ./bin/spark-shell [options]"
+export _SPARK_CMD_USAGE="Usage: ./bin/spark-shell [options]
+
+Scala REPL options:
+  -I <file>                   preload <file>, enforcing line-by-line interpretation"
 
 # SPARK-4161: scala does not assume use of the java classpath,
 # so we need to add the "-Dscala.usejavacp=true" flag manually. We

bin/spark-shell2.cmd

@@ -20,7 +20,13 @@ rem
 rem Figure out where the Spark framework is installed
 call "%~dp0find-spark-home.cmd"
 
-set _SPARK_CMD_USAGE=Usage: .\bin\spark-shell.cmd [options]
+set LF=^
+
+
+rem two empty lines are required
+set _SPARK_CMD_USAGE=Usage: .\bin\spark-shell.cmd [options]^%LF%%LF%^%LF%%LF%^
+Scala REPL options:^%LF%%LF%^
+  -I ^<file^>                   preload ^<file^>, enforcing line-by-line interpretation
 
 rem SPARK-4161: scala does not assume use of the java classpath,
 rem so we need to add the "-Dscala.usejavacp=true" flag manually. We