Update zh-CN tutorial translation

Update zh-CN tutorial translation

* etc/tutorials/TUTORIAL.cn (INTRO): Additions from English
version; say "interrupt" not "quit" a partially-entered command;
fixed minor inconsistency (English says partially-entered command,
Chinese used to say partially-executed command -- they are
different); converted half-width square brackets containing
translators' notes into full-width ones for consistency.
(BASIC CURSOR CONTROL): mention that a word boundary for Chinese
can also be space; fixed incorrect terminology (keyboard vs
terminal); retain English term "terminal"; retain English term
"prefix argument"; "most commands _interpret_ prefix args as
repeat-count", not "explain"; ensure space is added on both ends
of an English word.
(WINDOWS): Clarify it is the digit 1 in C-x 1.
(INSERTING AND DELETING): Mention the term "continuation line" and
retain its English term, because this term is mentioned again
later in the tutorial; avoid saying the name of "\\"; add
description for "electric" and a rough translation; sync English
for prefix argument for DEL and C-d; ensure that translators'
notes are marked explicitly so; add indentation to the instruction
about continuation lines; additional translations such as
mentioning C--, etc.
(FILES): Re-pluralize the heading; space around English words;
describe what C-g cancels (the command) when entering file name;
replace "search for" with "find"; minor rephrasing; explicit
translator notes; fixed the write-file message to no longer
contain ellipses (see src/fileio.c:5541).
(BUFFERS): Re-pluralize the heading; mention that C-x C-b also
shows buffer name; C-x s only looks for file-visiting buffers.
(EXTENDING THE COMMAND SET): Include C-x s and C-x b in list of
learned commands.
(MODE LINE): Remove the dashes in description on the point
location.
(MULTIPLE WINDOWS): Mention the difference between frame
and window; also add translator notes pointing to end of
tutorial which contains a terminology list.
(MULTIPLE FRAMES): Change the M-x commands to keybinds C-x 5 2 and
C-x 5 0 to sync with English.
(GETTING MORE HELP): Change C-h f to C-h x, to sync with English.
(INSTALLING PACKAGES): Rephrase "available packages", used to say
lit., "existing", now says lit., "installable"; consistently
translate the term "package", and retain its English name.
(TRANSLATION): Add my name as co-maintainer of this file.

* etc/tutorials/TUTORIAL.translators (TUTORIAL.cn): Add my name
as co-maintainer for TUTORIAL.cn.