docs: document the new --solosidescroll and 'set solosidescroll' options

These re-enable the historical jerky single-line sideways scrolling.

doc/nano.1

@@ -412,6 +412,14 @@ Furthermore, \fB^A\fR sets the mark,
 \fB^P\fR shows the position, \fB^T\fR goes to a line,
 \fB^W\fR writes out a file, and \fB^E\fR executes a command.
 Note that this overrides option \fB\-p\fR (\fB\-\-preserve\fR).
+.TP
+.BR \-1 ", " \-\-solosidescroll
+When the cursor approaches the right edge of the edit window, scroll only the
+current line sideways by a large amount, instead of scrolling all visible lines
+sideways by just the amount needed to keep the cursor in view.
+This latter whole-window smooth sideways scrolling is the new default behavior.
+The single-line jerky sideways scrolling enabled by this option was the old
+behavior \(em the only behavior that \fBnano\fR knew.
 
 .SH TOGGLES
 Several of the above options can be switched on and off also while

doc/nano.texi

@@ -789,6 +789,15 @@ Furthermore, @kbd{^A} sets the mark,
 @kbd{^W} writes out a file, and @kbd{^E} executes a command.
 Note that this overrides option @option{-p} (@option{--preserve}).
 
+@item -1
+@itemx --solosidescroll
+When the cursor approaches the right edge of the edit window, scroll only the
+current line sideways by a large amount, instead of scrolling all visible lines
+sideways by just the amount needed to keep the cursor in view.
+This latter whole-window smooth sideways scrolling is the new default behavior.
+The single-line jerky sideways scrolling enabled by this option was the old
+behavior --- the only behavior that @command{nano} knew.
+
 @end table
 
 @sp 1
@@ -1187,6 +1196,14 @@ Display lines that exceed the screen's width over multiple screen lines.
 (You can make this soft-wrapping occur at whitespace instead of rudely at
 the screen's edge, by using also @code{set atblanks}.)
 
+@item set solosidescroll
+When the cursor approaches the right edge of the edit window, scroll only the
+current line sideways by a large amount, instead of scrolling all visible lines
+sideways by just the amount needed to keep the cursor in view.
+This latter whole-window smooth sideways scrolling is the new default behavior.
+The single-line jerky sideways scrolling enabled by this option was the old
+behavior --- the only behavior that @command{nano} knew.
+
 @item set speller "@var{program} [@var{argument} @dots{}]"
 Use the given program to do spell checking and correcting.
 @xref{@option{--speller}} for details.

doc/nanorc.5

@@ -332,6 +332,14 @@ Display lines that exceed the screen's width over multiple screen lines.
 (You can make this soft-wrapping occur at whitespace instead of rudely at
 the screen's edge, by using also \fBset atblanks\fR.)
 .TP
+.B set solosidescroll
+When the cursor approaches the right edge of the edit window, scroll only the
+current line sideways by a large amount, instead of scrolling all visible lines
+sideways by just the amount needed to keep the cursor in view.
+This latter whole-window smooth sideways scrolling is the new default behavior.
+The single-line jerky sideways scrolling enabled by this option was the old
+behavior \(em the only behavior that \fBnano\fR knew.
+.TP
 .B set speller """\fIprogram\fR [\fIargument \fR...]\fB"""
 Use the given \fIprogram\fR to do spell checking and correcting, instead of
 using the built-in corrector that calls \fBhunspell\fR(1) or \fBspell\fR(1).

doc/sample.nanorc.in

@@ -171,6 +171,9 @@
 ## Spread overlong lines over multiple screen lines.
 # set softwrap
 
+## Scroll only the current line sideways (instead of all lines).
+# set solosidescroll
+
 ## Use this spelling checker instead of the internal one.  This option
 ## does not have a default value.
 # set speller "aspell -x -c"