In order to add more template types we need to update some copy from the current default templates provided.
This updates the `single` template copy as it's needed to clear up the ambiguity between the single template that is the fallback for any custom post type (including `post`) and the menu item to be added in a subsequent commit, that will allow the creation the `single-post` template and any `single_$post_type` template.
This commit backports the original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/41302#41302: Site Editor: Update single default template copy]
Follow-up to [52062], [52331].
Props ntsekouras, bernhard-reiter, jameskoster, Mamaduka, scruffian.
Fixes#56534.
git-svn-id: https://develop.svn.wordpress.org/trunk@54104 602fd350-edb4-49c9-b593-d223f7449a82