More updates for Buffers and Files chapters of Lisp manual.

More updates for Buffers and Files chapters of Lisp manual.

* doc/emacs/buffers.texi (Creating Buffers): Clarify that
generate-new-buffer uses generate-new-buffer-names.
(Killing Buffers): Remove bogus example duplicating buffer-live-p.

* doc/emacs/files.texi (Directory Names): Index entry for file name abbreviations.
(Relative File Names, File Name Expansion): Refer to it.
(Locating Files): Move locate-user-emacs-file documentation to
Standard File Names.
(Standard File Names): Add locate-user-emacs-file; update examples.