docs: add a paragraph on fingerprint generation (#12464)

* docs: add a paragraph on fingerprint generation

Since version 1.9.0+ changed the way Packer generates fingerprints for
HCP Packer, we add a small paragraph to explain how it used to be
generated before, and how it changed in this version.

* Apply suggestions from code review

---------

Co-authored-by: Wilken Rivera <wilken@hashicorp.com>

website/content/docs/hcp/index.mdx

@@ -47,6 +47,9 @@ Refer to [`hcp_packer_registry`](/packer/docs/templates/hcl_templates/blocks/bui
 
 Packer uses a unique fingerprint for tracking the completion of builds associated to an iteration. By default a fingerprint is automatically generated by Packer during each invocation of `packer build`, unless a fingerprint is manually provided via the `HCP_PACKER_BUILD_FINGERPRINT` environment variable.
 
+In versions before 1.9.0, this fingerprint was computed from the Git SHA of the current HEAD in which your template is stored. If you were running builds using a non Git managed template, you had to set the `HCP_PACKER_BUILD_FINGERPRINT` environment variable prior to invoking `packer build`.
+Starting with Packer 1.9.0, fingerprint generation does not rely on Git at all, and instead Packer now generates a Unique Lexicographically sortable Identifier (ULID) as the fingerprint for every `packer build` invocation.
+
 #### Fingerprints and Incomplete Iterations
 
 When you build a template with Packer, there's always a chance that it does not succeed because of a network issue, a provisioning failure, or some upstream error. When that happens, Packer will output the generated fingerprint associated with the incomplete iteration so that you can resume building that iteration using the `HCP_PACKER_BUILD_FINGERPRINT` environment variable; an iteration can be resumed until it is marked as complete. This environment variable is necessary for resuming an incomplete iteration, otherwise Packer will create a new iteration for the build.