bug#25196: mention how to see top or middle of each yank-pop

[積丹尼 Dan Jacobson]

To the bottom of
"yank-pop is an interactive compiled Lisp function in ‘simple.el’.
It is bound to M-y." help text, please add:

yank-pop is nice except when the popped text takes up more than a screen.

In that case the only way to see what is at the top of the text is to
move the cursor thus breaking the yank-pop sequence.

In such cases one might more or less salvage some text by looking directly for 
it in
the 'kill-ring', although it is in a different format than the original...

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Anyway the yank-pop docstring should at least mention 'kill-ring'.

Also in (info "(emacs) Earlier Kills") say how to see the top or middle
of each yank-pop without breaking the yank-pop sequence.

emacs-version "25.1.1"