docs: be more precise about when a dash is given instead of a filename

The dash doesn't have to be the first non-option argument: it can
be anywhere among the filenames -- there can even be multiple ones
(but don't mention this explicitly).

doc/nano.1

@@ -36,8 +36,8 @@ When giving a filename on the command line, the cursor can be put on a
 specific line by adding the line number with a plus sign (\fB+\fR) before
 the filename, and even in a specific column by adding it with a comma.
 
-If the first file specified is a dash (\fB\-\fR), \fBnano\fR will read
-data from standard input.
+As a special case: if instead of a filename a dash (\fB\-\fR) is given,
+\fBnano\fR will read data from standard input.
 
 .SH EDITING
 Entering text and moving around in a file is straightforward: typing the

doc/nano.texi

@@ -112,9 +112,9 @@ file (@pxref{Nanorc Files}).  And when using @code{set positionlog}
 (making @code{nano} remember the cursor position when you close a file),
 you will rarely need to specify a line number.
 
-As a special case: when the first file specified is a dash, @code{nano}
-will read data from standard input.  Which means you can pipe the output
-of a command straight into a buffer.
+As a special case: when instead of a filename a dash is given, @code{nano}
+will read data from standard input.  This means you can pipe the output of
+a command straight into a buffer, and then edit it.
 
 @node Command-line Options
 @chapter Command-line Options