Usability and documentation issues found trying to build an RC using dev-tools/scripts/buildAndPushRelease.py [LUCENE-6398]

This ticket is to address usability and documentation problems found while trying to do the 5.1 RC. Overall, it's a powerful process, but want to make it a bit easier for newbies doing it the first time.

Here's the start:

1) dev-tools/scripts/buildAndPushRelease.py

Prompts you for the following:

```Java
Enter GPG keystore password:
```

It's really asking for the passphrase the key you're using to sign the release artifacts, so it seems like it should prompt for something like:

```Java
Enter secret passphrase for key:
```

2. At the end of a release build, the scripts says:

```Java
Next set the PYTHON_EXEC env var and you can run the smoker tester:
$PYTHON_EXEC dev-tools/scripts/buildAndPushRelease.py URL
```

Running that command verbatim, you get an error:

```Java
usage: buildAndPushRelease.py [-h] [--no-prepare] [--push-remote USERNAME]
[--push-local PATH] [--sign KEYID]
[--rc-num NUM] [--smoke-test PATH]
checkout_path
buildAndPushRelease.py: error: Root path is not a valid lucene-solr checkout
[\~/dev/lw/projects/lucene_solr_5_1]$ $PYTHON_EXEC dev-tools/scripts/buildAndPushRelease.py --smoke-test file:///Users/timpotter/dev/lw/releases/5.1rc1/lucene-solr-5.1.0-RC1-rev1671632
usage: buildAndPushRelease.py [-h] [--no-prepare] [--push-remote USERNAME]
[--push-local PATH] [--sign KEYID]
[--rc-num NUM] [--smoke-test PATH]
checkout_path
buildAndPushRelease.py: error: the following arguments are required: checkout_path
```

So we need to fix the example to display what is actually needed to run.

Will add more to the docs as I go thru the process, but wanted to get a ticket in place for making improvements.

---
Migrated from [LUCENE-6398](https://issues.apache.org/jira/browse/LUCENE-6398) by Timothy Potter (@thelabdude), updated Apr 14 2019