syntax: man, groff: fix the string that introduces a comment

The comment marker is not «."» but «.\"», which requires three
backslashes to specify.  Also adjust the documentation.

doc/nano.texi

@@ -937,8 +937,8 @@ CSS files.  The characters before the pipe are prepended to the line and the
 characters after the pipe are appended at the end of the line.  If no pipe
 character is present, the entire string is prepended; for example, @t{"#"} for
 Python files.  If empty double quotes are specified, the comment/uncomment
-functions are disabled; for example, @t{""} for JSON.  Double quotes or backslashes
-may be escaped with a backslash; for example, @t{".\\""} for man page source.
+functions are disabled; for example, @t{""} for JSON.  Backslashes and double quotes
+should be escaped with a backslash; for example, @t{".\\\""} for man page source.
 The default value is @t{"#"}.
 
 @item color fgcolor,bgcolor "regex" @dots{}

doc/nanorc.5

@@ -335,8 +335,8 @@ CSS files.  The characters before the pipe are prepended to the line and the
 characters after the pipe are appended at the end of the line.  If no pipe
 character is present, the entire string is prepended; for example, "\fB#\fP" for
 Python files.  If empty double quotes are specified, the comment/uncomment
-function is disabled; for example, "" for JSON.  Double quotes or backslashes
-may be escaped with a backslash; for example, "\fB.\\"\fP" for man page source.
+function is disabled; for example, "" for JSON.  Backslashes and double quotes
+should be escaped with a backslash; for example, "\fB.\\\"\fP" for man page source.
 The default value is "\fB#\fP".
 .TP
 .B color \fIfgcolor\fR,\fIbgcolor\fR """\fIregex\fR""" ...

syntax/groff.nanorc

 ## Here is an example for groff.
 
 syntax "groff" "\.m[ems]$" "\.rof" "\.tmac$" "^tmac."
-comment ".\""
+comment ".\\\""
 
 # The argument of .ds or .nr
 color cyan "^\.(ds|nr) [^[:space:]]*"

syntax/man.nanorc

@@ -2,7 +2,7 @@
 
 syntax "man" "\.[1-9]x?$"
 magic "troff or preprocessor input text"
-comment ".\""
+comment ".\\\""
 
 color green "\.(SH|SS|TH) .*$"
 color brightgreen "\.(SH|SS|TH) " "\.([HIT]P)"