docs: explain the purpose of --markmatch / -^ / 'set markmatch'

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

doc/nano.1

@@ -337,6 +337,12 @@ 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 \-^ ", " \-\-markmatch
+After a search, set the mark at the end of the found match
+(if there is any) so that it gets highlighted.  This makes
+the match more visible, but also allows deleting the match
+with a single keystroke.
+.TP
 .BR \-_ ", " \-\-minibar
 Suppress the title bar and instead show information about
 the current buffer at the bottom of the screen, in the space

doc/nano.texi

@@ -477,6 +477,13 @@ 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 -^
+@itemx --markmatch
+After a search, set the mark at the end of the found match
+(if there is any) so that it gets highlighted.  This makes
+the match more visible, but also allows deleting the match
+with a single keystroke.
+
 @item -_
 @itemx --minibar
 Suppress the title bar and instead show information about
@@ -919,6 +926,12 @@ try using libmagic to determine the applicable syntax.
 (Calling libmagic can be relatively time consuming.
 It is therefore not done by default.)
 
+@item set markmatch
+After a search, set the mark at the end of the found match
+(if there is any) so that it gets highlighted.  This makes
+the match more visible, but also allows deleting the match
+with a single keystroke.
+
 @anchor{@code{set matchbrackets}}
 @item set matchbrackets "@var{characters}"
 Specify the opening and closing brackets that can be found by bracket

doc/nanorc.5

@@ -176,6 +176,12 @@ try using libmagic to determine the applicable syntax.
 (Calling libmagic can be relatively time consuming.
 It is therefore not done by default.)
 .TP
+.B set markmatch
+After a search, set the mark at the end of the found match
+(if there is any) so that it gets highlighted.  This makes
+the match more visible, but also allows deleting the match
+with a single keystroke.
+.TP
 .BI "set matchbrackets """ characters """
 Specify the opening and closing brackets that can be found by bracket
 searches.  This may not include blank characters.  The opening set must

doc/sample.nanorc.in

@@ -79,6 +79,9 @@
 ## Fall back to slow libmagic to try and determine an applicable syntax.
 # set magic
 
+# After a search, set the mark at the end of the found match (if any).
+# set markmatch
+
 ## The opening and closing brackets that can be found by bracket
 ## searches.  They cannot contain blank characters.  The former set must
 ## come before the latter set, and both must be in the same order.