Revise documentation concerning selections outside X

Revise documentation concerning selections outside X

* doc/lispref/frames.texi (Accessing Selections, X Selections):
Correct markup, averting the recognition of X followed by a
sentence stop as an acronym.
(Other Selections): Relate the nature of the MS-Windows
selection emulation, how it functions, and its deficiencies.
Clarify paragraphs concerning PGTK and Nextstep.

* doc/misc/efaq.texi (Emacs in a Linux console):

* doc/misc/use-package.texi (Conditional loading): Correct
markup, averting the recognition of X followed by a sentence
stop as an acronym.