www: document the reason for simulated NVRAM

It is probably not obvious why the simulated NVRAM is present, so
explain.

esp32/www/abcmem.html

@@ -25,6 +25,9 @@
 	  <input is="x-box" name="abc.mem.abc80.nvram.22k" />
 	  <span class="help">22-24K (incompatible with TKN80)</span>
 	</label>
+	<p class="help">If simulated NVRAM is present, it is used for
+	UFD-DOS and PUN80 system variables; otherwise up to 320
+	bytes of BASIC memory may be consumed.</p>
 	<label class="ufddos">
 	  <b>UFD-DOS ROM</b>
 	  <input is="x-box" name="abc.mem.abc80.ufddos" />