docs: add 'set linenumbers' and '--linenumbers' to the texinfo document

Also mention the corresponding toggle (M-#).

doc/texinfo/nano.texi

@@ -270,6 +270,10 @@ the previous line.
 Make the 'Cut Text' command (normally ^K) cut from the current cursor
 position to the end of the line, instead of cutting the entire line.
 
+@item -l
+@itemx --linenumbers
+Display line numbers to the left of the text area.
+
 @item -m
 @itemx --mouse
 Enable mouse support, if available for your system.  When enabled, mouse
@@ -592,6 +596,9 @@ toggles the @code{-z} (@code{--suspend}) command-line option.
 @item Soft Wrapping Toggle (Meta-$)
 toggles the @code{-$} (@code{--softwrap}) command-line option.
 
+@item Line Numbers Toggle (Meta-#)
+toggles the @code{-l} (@code{--linenumbers}) command-line option.
+
 @end table
 
 
@@ -700,6 +707,9 @@ Specify the color combination to use for the shortcut key combos
 in the two help lines at the bottom of the screen.
 See @code{set titlecolor} for more details.
 
+@item set linenumbers
+Display line numbers to the left of the text area.
+
 @item set locking
 Enable vim-style lock-files for when editing files.