Bug 1769581: Update help icon text for topic selection

Mention that the subscribe / unsubscribe options are
only available when not subscribed to the forum.

behatnotneeded

Change-Id: I5c34aed233a75a9e26346df69a52137518e44218

htdocs/interaction/forum/lang/en.utf8/help/sections/updatemod.html

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 <h3>Update</h3>
-<p>Select topics by checking the box to the left of each topic, then choose an action to update the selected topics.</p>
-<p><strong>Subscribe</strong>: You will receive notifications for posts to selected topics.</p>
-<p><strong>Unsubscribe</strong>: You will not receive notifications for posts to selected topics.</p>
+<p>Select topics by checking the box to the left of each topic. Then choose an action to update the selected topics.</p>
+<p><strong>Subscribe</strong>: You will receive notifications for posts to selected topics. This option is only available if you are not already subscribed to the entire forum.</p>
+<p><strong>Unsubscribe</strong>: You will not receive notifications for posts to selected topics. This is only possible when you are not subscribed to the forum.</p>
 <p><strong>Sticky</strong>: Selected topics will appear at the top of every page.</p>
 <p><strong>Unsticky</strong>: Selected topics will no longer appear at the top of every page.</p>
 <p><strong>Open</strong>: All group members will be able to post to selected topics.</p>
 <p><strong>Close</strong>: Only the group administrators and forum moderators will be able to post to selected topics.</p>
-<p><strong>Move to</strong>: Only the group administrators and forum moderators will be able to move selected topics to another forum.</p>
+<p><strong>Move to</strong>: Only the group administrators and forum moderators will be able to move selected topics to another forum if another forum exists.</p>