Update docs to include new --existing-resource-policy flag

[a-mccarthy]

From comment
Issue link: #4842
Design link: #4613
Implementation link: #4628

[a-mccarthy]

The original PR added the setting into the restore API docs, but since we are making updates to the restore docs as well in #4642 and #4150, it think its also worth adding this functionality in.

[reasonerjt]

This has been documented in the restore reference:
https://velero.io/docs/main/restore-reference/#restore-existing-resource-policy
as well as the API documentation:
https://velero.io/docs/main/api-types/restore/

[reasonerjt]

closing this issue as it's been fixed.