docs: add a missing double quote in the default brackets string

It was accidentally lost three years ago, in commit 7e1bc82d.

Also improve the description of this option in the other documents.

doc/nano.texi

@@ -735,7 +735,7 @@ Set the characters treated as closing brackets when justifying
 paragraphs.  This may not include blank characters.  Only closing
 punctuation (see @code{set punct}), optionally followed by the specified
 closing brackets, can end sentences.  The default value is
-@t{"')>]@}"}.
+@t{""')>]@}"}.
 
 @item set casesensitive
 Do case-sensitive searches by default.

doc/nanorc.5

@@ -93,7 +93,7 @@ setting the options \fBtitlecolor\fP, \fBstatuscolor\fP, \fBkeycolor\fP,
 .B set brackets "\fIstring\fP"
 Set the characters treated as closing brackets when justifying
 paragraphs.  This may not include blank characters.  Only closing
-punctuation (see \fBpunct\fP), optionally followed by the specified
+punctuation (see \fBset punct\fP), optionally followed by the specified
 closing brackets, can end sentences.  The default value is "\fB"')>]}\fP".
 .TP
 .B set casesensitive

doc/sample.nanorc.in

@@ -37,10 +37,9 @@
 ## Use bold text instead of reverse video text.
 # set boldtext
 
-## The characters treated as closing brackets when justifying
+## The characters treated as closing brackets when justifying paragraphs.
-## paragraphs.  They cannot contain blank characters.  Only closing
+## This may not include any blank characters.  Only closing punctuation,
-## punctuation, optionally followed by closing brackets, can end
+## optionally followed by these closing brackets, can end sentences.
-## sentences.
 # set brackets ""')>]}"
 
 ## Do case-sensitive searches by default.