Prefer saying "Info manual" to "info page" in docs

Prefer saying "Info manual" to "info page" in docs

Pointed out by Eli Zaretskii in:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=39215#14
* doc/emacs/Makefile.in:
* doc/lispintro/Makefile.in:
* doc/lispref/Makefile.in:
* doc/misc/Makefile.in:
* lisp/dired-x.el (top-level):
* lisp/gnus/gnus-sum.el (gnus-summary-mode):
* lisp/progmodes/cperl-mode.el (cperl-info-page): Doc fix; prefer
saying "Info manual" over "info page".