docs: explain what the options --stateflags (-%) and 'set stateflags' do

Also, colorize 'set stateflags' as valid option in a nanorc file.

doc/nano.1

@@ -95,6 +95,14 @@ The default key bindings can be changed via a \fInanorc\fR file -- see
 
 .SH OPTIONS
 .TP
+.BR \-% ", " \-\-stateflags
+Use the top-right corner of the screen for showing some state flags:
+\fBI\fR when auto-indenting, \fBM\fR when the mark is on, \fBL\fR when
+hard-wrapping (breaking long lines), \fBR\fR when recording a macro,
+and \fBS\fR when soft-wrapping.
+When the buffer is modified, a star (\fB*\fR) is shown after the
+filename in the center of the title bar.
+.TP
 .BR \-A ", " \-\-smarthome
 Make the Home key smarter.  When Home is pressed anywhere but at the
 very beginning of non-whitespace characters on a line, the cursor will

doc/nano.texi

@@ -165,6 +165,15 @@ a command straight into a buffer, and then edit it.
 
 @table @option
 
+@item -%
+@itemx --stateflags
+Use the top-right corner of the screen for showing some state flags:
+@code{I} when auto-indenting, @code{M} when the mark is on, @code{L} when
+hard-wrapping (breaking long lines), @code{R} when recording a macro,
+and @code{S} when soft-wrapping.
+When the buffer is modified, a star (@code{*}) is shown after the
+filename in the center of the title bar.
+
 @item -A
 @itemx --smarthome
 Make the Home key smarter.  When Home is pressed anywhere but at the
@@ -1008,6 +1017,14 @@ the screen's edge, by using also @code{set atblanks}.)
 Use the given program to do spell checking and correcting.
 @xref{@option{--speller}} for details.
 
+@item set stateflags
+Use the top-right corner of the screen for showing some state flags:
+@code{I} when auto-indenting, @code{M} when the mark is on, @code{L} when
+hard-wrapping (breaking long lines), @code{R} when recording a macro,
+and @code{S} when soft-wrapping.
+When the buffer is modified, a star (@code{*}) is shown after the
+filename in the center of the title bar.
+
 @item set statuscolor [bold,][italic,]@var{fgcolor},@var{bgcolor}
 Use this color combination for the status bar.
 @xref{@code{set functioncolor}} for valid color names.

doc/nanorc.5

@@ -294,6 +294,14 @@ the screen's edge, by using also \fBset atblanks\fR.)
 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).
 .TP
+.B set stateflags
+Use the top-right corner of the screen for showing some state flags:
+\fBI\fR when auto-indenting, \fBM\fR when the mark is on, \fBL\fR when
+hard-wrapping (breaking long lines), \fBR\fR when recording a macro,
+and \fBS\fR when soft-wrapping.
+When the buffer is modified, a star (\fB*\fR) is shown after the
+filename in the center of the title bar.
+.TP
 .B set  statuscolor \fR[\fBbold,\fR][\fBitalic,\fR]\fIfgcolor\fB,\fIbgcolor\fR
 Specify the color combination to use for the status bar.
 See \fBset titlecolor\fR for more details.

doc/sample.nanorc.in

@@ -159,6 +159,10 @@
 ## does not have a default value.
 # set speller "aspell -x -c"
 
+## Use the end of the title bar for some state flags: I = auto-indenting,
+## M = mark, L = hard-wrapping long lines, R = recording, S = soft-wrapping.
+# set stateflags
+
 ## Allow nano to be suspended (with ^Z by default).
 # set suspendable
 ## (The old form of this option, 'set suspend', is deprecated.)

syntax/nanorc.nanorc

@@ -7,7 +7,7 @@ comment "#"
 color brightred ".*"
 
 # Keywords
-color brightgreen "^[[:space:]]*(set|unset)[[:space:]]+(afterends|allow_insecure_backup|atblanks|autoindent|backup|boldtext|breaklonglines|casesensitive|constantshow|cutfromcursor|emptyline|historylog|indicator|jumpyscrolling|linenumbers|locking|mouse|multibuffer|noconvert|nohelp|nonewlines|positionlog|preserve|quickblank|rawsequences|rebinddelete|regexp|saveonexit|showcursor|smarthome|softwrap|suspendable|tabstospaces|trimblanks|unix|view|wordbounds|zap)\>"
+color brightgreen "^[[:space:]]*(set|unset)[[:space:]]+(afterends|allow_insecure_backup|atblanks|autoindent|backup|boldtext|breaklonglines|casesensitive|constantshow|cutfromcursor|emptyline|historylog|indicator|jumpyscrolling|linenumbers|locking|mouse|multibuffer|noconvert|nohelp|nonewlines|positionlog|preserve|quickblank|rawsequences|rebinddelete|regexp|saveonexit|showcursor|smarthome|softwrap|stateflags|suspendable|tabstospaces|trimblanks|unix|view|wordbounds|zap)\>"
 color yellow "^[[:space:]]*set[[:space:]]+((error|function|key|number|scroller|selected|status|stripe|title)color)[[:space:]]+(bold,)?(italic,)?(bright|light)?(white|black|red|blue|green|yellow|magenta|cyan|normal|pink|purple|mauve|lagoon|mint|lime|peach|orange|latte)?(,(light)?(white|black|red|blue|green|yellow|magenta|cyan|normal|pink|purple|mauve|lagoon|mint|lime|peach|orange|latte))?\>"
 color brightgreen "^[[:space:]]*set[[:space:]]+(backupdir|brackets|errorcolor|functioncolor|keycolor|matchbrackets|numbercolor|operatingdir|punct|quotestr|scrollercolor|selectedcolor|speller|statuscolor|stripecolor|titlecolor|whitespace|wordchars)[[:space:]]+"
 color brightgreen "^[[:space:]]*set[[:space:]]+(fill[[:space:]]+-?[[:digit:]]+|(guidestripe|tabsize)[[:space:]]+[1-9][0-9]*)\>"