This makes the line-numbers toggle rebindable, like all the others.

And remove a detail that is untrue when 'set selectedcolor' is used.

The new option 'set errorcolor' allows the user to specify the color
combination for the status bar when an error message is displayed.

This addresses https://savannah.gnu.org/bugs/?48992.

This brings it in line with the other three "flip" toggles.

After commit e739448c, macros should work also when ncurses produces
escape sequences instead of keycodes.

The option now causes nano to trim trailing whitespace also when
hardwrapping occurs while the user is typing.

If the user uses a single version of nano, they have no need for
--quiet.  If they do sometimes use an older version and don't want
to see the warnings, they can use 2>/dev/null (they could make an
alias for that and put it before the call of nano).

List "browser", "pageup" and "pagedown" in the manuals; the first
because it matches "speller", and the latter two because they are
what these functions are commonly known by.  The names "tofiles",
"prevpage" and "nextpage" are now undocumented, but they are still
recognized by the code, as they match the tags for these functions.

The new option 'set selectedcolor' applies to marked text, to the
currently selected file in the file browser, and to the highlighted
match during interactive search-and-replace.

And reword them too, so that it is clearer what the mechanism is
for determining which syntax to use for a file.

Rename the corresponding rc-file option too, of course.

And in the bargain improve some mark-up here and there.

And rewrap the feature toggles, so that a title does not start
a sentence.

And refer backward instead of forward.