docs: mention that "Ins" and "Del" are valid rebindable keys

Also, consistently refer to Latin letters and ASCII characters
where needed, and document more accepted characters.

This addresses https://savannah.gnu.org/bugs/?54071.

doc/nano.texi

@@ -1078,16 +1078,20 @@ The format of @code{key} should be one of:
 
 @itemize @w{}
 @item
-@code{^}@  followed by an ASCII character or the word "Space".
+@code{^}@  followed by a Latin letter, one of several ASCII characters
+(@, ], \, ^, or _), or the word "Space".
 Example: @code{^C}.
 
 @item
-@code{M-}@  followed by a ASCII character or the word "Space".
+@code{M-}@  followed by any ASCII character except [, or the word "Space".
 Example: @code{M-C}.
 
 @item
 @code{F}@  followed by a numeric value from 1 to 16.
 Example: @code{F10}.
+
+@item
+the word "Ins" or the word "Del".
 @end itemize
 
 @sp 1

doc/nanorc.5

@@ -427,16 +427,19 @@ The format of \fIkey\fP should be one of:
 .RS 3
 .TP 3
 .B ^
-followed by an alpha character or the word "Space".
+followed by a Latin letter, one of several ASCII characters
+(@, ], \, ^, or _), or the word "Space".
 Example: ^C
 .TP
 .B M-
-followed by a printable character or the word "Space".
+followed by any ASCII character except [, or the word "Space".
 Example: M-C
 .TP
 .B F
 followed by a numeric value from 1 to 16.
 Example: F10
+.TP
+the word "Ins" or the word "Del".
 .RE
 
 .TP