docs: mention that "normal" can be used to give things the default color

doc/nano.texi

@@ -1001,8 +1001,11 @@ The default value is @t{"#"}.
 Display all pieces of text that match the
 extended regular expression "regex" with foreground color "fgcolor" and
 background color "bgcolor", at least one of which must be specified.
-Valid colors for foreground and background are: white, black, red,
-blue, green, yellow, magenta, and cyan.  You may use the prefix "bright"
+Valid names for foreground and background color are:
+@code{white}, @code{black}, @code{blue}, @code{green},
+@code{red}, @code{cyan}, @code{yellow}, @code{magenta}, and @code{normal}
+--- where @code{normal} means the default foreground or background color.
+You may use the prefix @code{bright}
 to get a stronger color highlight for the foreground.  If your
 terminal supports transparency, not specifying a "bgcolor" tells @command{nano}
 to attempt to use a transparent background.

doc/nanorc.5

@@ -356,9 +356,11 @@ The default value is "\fB#\fP".
 Display all pieces of text that match
 the extended regular expression \fIregex\fP with foreground color
 \fIfgcolor\fP and background color \fIbgcolor\fP, at least one of which
-must be specified.  Valid colors for foreground and background
-are: white, black, red, blue, green, yellow, magenta, and cyan.  You may
-use the prefix "bright" to get a stronger color highlight for the
+must be specified.  Valid names for foreground and background colors are:
+.BR white ", " black ", " blue ", " green ", " red ", "
+.BR cyan ", " yellow ", " magenta ", and " normal
+-- where \fBnormal\fR means the default foreground or background color.
+You may use the prefix \fBbright\fR to get a stronger color highlight for the
 foreground.  If your terminal supports transparency, not specifying a
 \fIbgcolor\fP tells \fBnano\fP to attempt to use a transparent
 background.