Document git_email in databricks_git_credential resource (#5099)

## Changes
<!-- Summary of your changes that are easy to understand -->

`git_username` is required when `git_email` is specified, but `git_email
isn't required with `git_username`. However Terraform SDKv2 doesn't
allow to express this kind of dependency, so I didn't modify the code
(it should be possible with the Plugin Framework).

## Tests
<!--
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->

- [ ] `make test` run locally
- [x] relevant change in `docs/` folder
- [ ] covered with integration tests in `internal/acceptance`
- [ ] using Go SDK
- [ ] using TF Plugin Framework
- [x] has entry in `NEXT_CHANGELOG.md` file

Author: alexott

NEXT_CHANGELOG.md

@@ -10,6 +10,8 @@
 
 ### Documentation
 
+* Document `git_email` in `databricks_git_credential` resource ([#5099](https://github.com/databricks/terraform-provider-databricks/pull/5099))
+
 ### Exporter
 
 ### Internal Changes

docs/resources/git_credential.md

@@ -36,7 +36,8 @@ resource "databricks_git_credential" "ado" {
 The following arguments are supported:
 
 * `personal_access_token` - (Optional, required for some Git providers) The personal access token used to authenticate to the corresponding Git provider. If value is not provided, it's sourced from the first environment variable of [`GITHUB_TOKEN`](https://registry.terraform.io/providers/integrations/github/latest/docs#oauth--personal-access-token), [`GITLAB_TOKEN`](https://registry.terraform.io/providers/gitlabhq/gitlab/latest/docs#required), or [`AZDO_PERSONAL_ACCESS_TOKEN`](https://registry.terraform.io/providers/microsoft/azuredevops/latest/docs#argument-reference), that has a non-empty value.
-* `git_username` - (Optional, required for some Git providers) user name at Git provider.
+* `git_username` - (Optional, required for some Git providers or when `git_email` is specified) user name at Git provider.  For most Git providers it is only used to set the Git committer & author names for commits, however it may be required for authentication depending on your Git provider / token requirements.
+* `git_email` - (Optional, required for some Git providers) The email associated with your Git provider user account. Used for authentication with the remote repository and also sets the author & committer identity for commits.
 * `git_provider` -  (Required) case insensitive name of the Git provider.  Following values are supported right now (could be a subject for a change, consult [Git Credentials API documentation](https://docs.databricks.com/dev-tools/api/latest/gitcredentials.html)): `gitHub`, `gitHubEnterprise`, `bitbucketCloud`, `bitbucketServer`, `azureDevOpsServices`, `gitLab`, `gitLabEnterpriseEdition`, `awsCodeCommit`, `azureDevOpsServicesAad`.
 * `is_default_for_provider` - (Optional) boolean flag specifying if the credential is the default for the given provider type.
 * `name` - (Optional) the name of the git credential, used for identification and ease of lookup.