* lisp/info.el (Info-read-node-name-1): Allow empty node name in (FILENAME)

* lisp/info.el (Info-read-node-name-1): Allow empty node name in (FILENAME)
that defaults to the Top node.
(Info-goto-node, Info-read-node-name): Doc fix to mention that
the short format (FILENAME) goes to the Top node.
(Info-build-node-completions): Rename arg `file' to `filename'.

* doc/misc/info.texi (Go to node): Mention the abbreviated format
`(FILENAME)' equal to `(FILENAME)Top'.