docs: mention that ^[ (Esc) is unbindable, and explain why

doc/nano.texi

@@ -1186,6 +1186,8 @@ with Shift.)
 @end itemize
 
 Rebinding @code{^M} (Enter) or @code{^I} (Tab) is probably not a good idea.
+Rebinding @code{^[} (Esc) is not possible, because its keycode
+is the starter byte of escape sequences and Meta keystrokes.
 On some terminals it's not possible to rebind @code{^H} (unless @code{--raw}
 is used) because its keycode is identical to that of the Backspace key.
 

doc/nanorc.5

@@ -509,6 +509,8 @@ Example: F10.
 .RE
 
 Rebinding \fB^M\fR (Enter) or \fB^I\fR (Tab) is probably not a good idea.
+Rebinding \fB^[\fR (Esc) is not possible, because its keycode
+is the starter byte of escape sequences and Meta keystrokes.
 On some terminals it's not possible to rebind \fB^H\fR (unless \fB\-\-raw\fR
 is used) because its keycode is identical to that of the Backspace key.