RootCommand.cs: Added info about missing option --outdir. (#14188)

Added info about missing option `--outdir`.

This works in my testing ( #14186 ), but was not outputted when doing
`bicep --help`.

# Contributing a Pull Request

If you haven't already, read the full [contribution
guide](https://github.com/Azure/bicep/blob/main/CONTRIBUTING.md). The
guide may have changed since the last time you read it, so please
double-check. Once you are done and ready to submit your PR, run through
the relevant checklist below.

## Contributing to documentation

* [ ] All documentation contributions should be made directly in the
[Bicep documentation on Microsoft
Docs](https://docs.microsoft.com/azure/azure-resource-manager/bicep/).

## Contributing an example

We are integrating the Bicep examples into the [Azure QuickStart
Templates](https://github.com/Azure/azure-quickstart-templates/blob/master/1-CONTRIBUTION-GUIDE/README.md).
If you'd like to contribute new example `.bicep` files that showcase
abilities of the language, please follow [these
instructions](https://github.com/Azure/azure-quickstart-templates/blob/master/1-CONTRIBUTION-GUIDE/README.md)
to add them directly there. We can still take bug reports and fixes for
the existing examples for the time being.

* [ ] This is a bug fix for an existing example
* [ ] I have resolved all warnings and errors shown by the Bicep VS Code
extension
* [ ] I have checked that all tests are passing by running `dotnet test`
* [ ] I have consistent casing for all of my identifiers and am using
camelCasing unless I have a justification to use another casing style

## Contributing a feature

* [ ] I have opened a new issue for the proposal, or commented on an
existing one, and ensured that the Bicep maintainers are good with the
design of the feature being implemented
* [ ] I have included "Fixes #{issue_number}" in the PR description, so
GitHub can link to the issue and close it when the PR is merged
* [ ] I have appropriate test coverage of my new feature

## Contributing a snippet

* [ ] I have a snippet that is either a single, generic resource or
multi resource that uses [parent-child
syntax](https://docs.microsoft.com/azure/azure-resource-manager/bicep/child-resource-name-type)
* [ ] I have checked that there is not an equivalent snippet already
submitted
* [ ] I have used camelCasing unless I have a justification to use
another casing style
* [ ] I have placeholders values that correspond to their property names
(e.g. `dnsPrefix: 'dnsPrefix'`), unless it's a property that MUST be
changed or parameterized in order to deploy. In that case, I use
'REQUIRED' e.g.
[keyData](./src/Bicep.LangServer/Files/SnippetTemplates/res-aks-cluster.bicep#L26)
* [ ] I have my symbolic name as the first tab stop ($1) in the snippet.
e.g.
[res-aks-cluster.bicep](./src/Bicep.LangServer/Files/SnippetTemplates/res-aks-cluster.bicep)
* [ ] I have a resource name property equal to "name"
* [ ] If applicable, I have set the `location` property to `location:
/*${<id>:location}*/'location'` (not `resourceGroup().location`) where
`<id>` is a placeholder id, and added `param location string` to the
test's main.bicep file so that the resulting main.combined.bicep file
used in the tests compiles without errors
* [ ] I have verified that the snippet deploys correctly when used in
the context of an actual bicep file

  e.g.

  ```bicep
resource aksCluster
'Microsoft.ContainerService/managedClusters@2021-03-01' = {
    name: 'name'
  ```

src/Bicep.Cli/Commands/RootCommand.cs

@@ -213,14 +213,16 @@ Builds a .json file from a .bicepparam file.
 
     Options:
       --bicep-file <file>            Verifies if the specified bicep file path matches the one provided in the params file using declaration
-      --outfile <file>               Saves the param output json as the specified file path.
-      --stdout                       Prints the param and bicep json output to stdout.
+      --outdir <dir>                 Saves the output of building the parameter file only (.bicepparam) as json to the specified directory.
+      --outfile <file>               Saves the output of building the parameter file only (.bicepparam) as json to the specified file path.
+      --stdout                       Prints the output of building both the parameter file (.bicepparam) and the template it points to (.bicep) as json to stdout.
       --no-restore                   Builds the bicep file (referenced in using declaration) without restoring external modules.
       --diagnostics-format <format>  Sets the format with which diagnostics are displayed. Valid values are ( {string.Join(" | ", Enum.GetNames(typeof(DiagnosticsFormat)))} ).
 
     Examples:
       bicep build-params params.bicepparam
       bicep build-params params.bicepparam --stdout
+      bicep build-params params.bicepparam --outdir dir1
       bicep build-params params.bicepparam --outfile otherParams.json
       bicep build-params params.bicepparam --no-restore
       bicep build-params params.bicepparam --diagnostics-format sarif