docs: mention that a negative number after "+" counts from the end

A negative line number given on the command line counts from the end
of the relevant file (-1 meaning the first line from the bottom),
and a negative column number from the end of the relevant line.

The feature was added in version 2.8.5 but not mentioned in the manual.

This addresses https://savannah.gnu.org/bugs/?57342.
Suggested-by: Andreas Schamanek <schamane@fam.tuwien.ac.at>

doc/nano.1

@@ -44,6 +44,7 @@ syntax coloring, line numbering, and soft-wrapping overlong lines.
 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.
+(Negative numbers count from the end of the file or line.)
 The cursor can be put on the first or last occurrence of a specific string
 by specifying that string after \fB+/\fR or \fB+?\fR before the filename.
 The string can be made case sensitive and/or caused to be interpreted as a

doc/nano.texi

@@ -128,6 +128,7 @@ section), and to edit several files in a row.  Additionally, the cursor
 can be put on a specific line of a file by adding the line number
 with a plus sign before the filename, and even in a specific column by
 adding it with a comma.
+(Negative numbers count from the end of the file or line.)
 The cursor can also be put on the first or last occurrence of a specific string
 by specifying that string after @code{+/} or @code{+?} before the filename.
 The string can be made case sensitive and/or caused to be interpreted as a