* doc/emacs/package.texi (Specifying Package Sources): List the
accepted keys in a new subsection of Fetching Package Sources.
* lisp/emacs-lisp/package-vc.el (package-vc-selected-packages):
Mention the `:doc` key. Add the `:doc` key to the Customize form,
mention the new Info node, correct "TexInfo" to "Texinfo", avoid
Git-specific terms for the description of `:branch`, mention guessing
`:vc-backend` based on the URL.
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