Refine the help text wording for backend editor

[timse201]

actually we use the wording "The top section controls the editor within the dashboard" for the help text on wp-admin/options-writing.php:24. That's not very precise and confuses newcomers.

fixes: https://core.trac.wordpress.org/ticket/65214

[sabernhardt]

[19507] changed "within these administration screens" to "within the dashboard"; however,

• ​HelpHub's article uses the term "Administration Screen" and
• Ticket 63048 avoided "dashboard" in favor of "administration" when referring to administration color schemes.

I am not sure "backend editor" would help.

• I did not find "backend" (or "back end" or "back-end") in any user-facing strings.
• The post editor technically can be added on the front end.

Something like "the interface you use when adding and editing posts" could describe the post editor (I pulled that from the first two sentences in the ​Writing Settings article).

Other parts of the string could be edited, too.

• Both Post via Email and Update Services can be removed via filter. Without either of those displaying, "top section" and "the rest" would not fit.
• When real-time collaboration has an opt-in setting (#64904), the help text might mention that.