help: Update superscript page to 3.38

Update usage instructions to specify the modes in which the superscript and
subscript buttons are available; Update grammar to be more consistent with
other help pages.

Ref: #185

help/C/superscript.page

@@ -3,6 +3,14 @@
       id="superscript">
 
     <info>
+        <revision pkgversion="3.38.0" date="2020-09-25" status="review"/>
+
+        <credit type="editor copyright">
+            <name>Pranali Deshmukh</name>
+            <email>pranali21293@gmail.com</email>
+            <years>2020</years>
+        </credit>
+
         <!-- Place this at the end of the section -->
         <title type="sort">_</title>
         <link type="guide" xref="index#ui"/>
@@ -19,15 +27,14 @@
     </p>
     </example>
     <p>
-    To enter superscript numbers with the mouse select the number mode using the the <gui>↑n</gui> and <gui>↓n</gui> buttons.
+    To enter superscript numbers with the mouse select the number mode using the the <gui>↑n</gui> and <gui>↓n</gui> buttons (available in the <gui>Advanced Mode</gui> and the <gui>Programming Mode</gui>).
     When one of these modes is active clicking the number buttons will enter numbers in superscript or subscript.
-    To return to normal number mode click the active button.
+    To return to normal number mode click the active number button.
     </p>
     <p>
-    To enter superscript numbers with the keyboard hold down <key>Ctrl</key> while entering the number.
-    Hold <key>Alt</key> for subscript.
+    To enter superscript numbers use <keyseq><key>Ctrl</key>number</keyseq>, for subscript numbers use <keyseq><key>Alt</key>number</keyseq>. 
     </p>
     <p>
-    The number mode returns to normal when entering the next non-number character (e.g. +).
+    The number mode returns to normal when entering the next non-number character (e.g. +, -).
     </p>
 </page>