docs: mention that syntax definitions are available in /usr/share/nano/

Inspired-by: Hussam al-Homsi <sawuare@gmail.com>

doc/nano.1

@@ -350,6 +350,9 @@ exists), either \fI~/.nanorc\fR or \fI$XDG_CONFIG_HOME/nano/nanorc\fR
 or \fI~/.config/nano/nanorc\fR, whichever is encountered first.  See
 .BR nanorc (5)
 for more information on the possible contents of those files.
+.sp
+See \fI/usr/share/nano/\fR and \fI/usr/share/nano/extra/\fR
+for available syntax-coloring definitions.
 
 .SH NOTES
 If no alternative spell checker command is specified on the command

doc/nano.texi

@@ -1076,6 +1076,9 @@ powerful enough to fully parse a file.  Nevertheless, regular
 expressions can do a lot and are easy to make, so they are a
 good fit for a small editor like @command{nano}.
 
+See @file{/usr/share/nano/} and @file{/usr/share/nano/extra/}
+for the syntax-coloring definitions that are available out of the box.
+
 All regular expressions in @command{nano} are POSIX extended regular expressions
 (ERE).  This means that @code{.}, @code{?}, @code{*}, @code{+}, @code{^},
 @code{$}, and several other characters are special.

doc/nanorc.5

@@ -965,6 +965,10 @@ System-wide configuration file.
 .TP
 .IR ~/.nanorc " or " $XDG_CONFIG_HOME/nano/nanorc " or " ~/.config/nano/nanorc
 Per-user configuration file.
+.TP
+.I /usr/share/nano/*
+Syntax definitions for the syntax coloring of common file types
+(and for less common file types in the \fIextra/\fR subdirectory).
 
 .SH SEE ALSO
 .BR nano (1)