docs: fediverse usernames can't contain Unicode or start or end with …

…_, -, ~

for #982

templates/docs.html

@@ -310,9 +310,10 @@ <h3 id="from-fediverse">From the fediverse ⚠️ BETA ⚠️</h3>
 <p><em>⚠️ Bridging from the fediverse to Bluesky is currently in early beta testing. Expect bugs, incomplete features, downtime, and other rough edges! ⚠️</em></p>
 <p>To bridge your fediverse account into Bluesky and interact with people there, search for and follow <code>@[email protected]</code>.</p>
 <p>If your fediverse account is <code>@[user]@[instance]</code>, your bridged account will have the handle <code>[user].[instance].ap.brid.gy</code> in Bluesky. For example, <a href="https://indieweb.social/@snarfed">@[email protected]</a> is bridged into Bluesky as <a href="https://bsky.app/profile/snarfed.indieweb.social.ap.brid.gy">@snarfed.indieweb.social.ap.brid.gy</a>.</p>
-<p>If your fediverse username has <code>_</code> (underscore) or <code>~</code> (tilde) characters, they're converted to <code>-</code> (dash) characters. For example, @[email protected] is bridged into Bluesky as @a-b-c.my.social.ap.brid.gy.
+<p>If your fediverse username has <code>_</code> (underscore) or <code>~</code> (tilde) characters, they're converted to <code>-</code> (dash) characters. For example, <code>@[email protected]</code> is bridged into Bluesky as <code>@a-b-c.my.social.ap.brid.gy</code>.</p>
+<p><em>Fediverse usernames that begin or end with <code>_</code>, <code>-</code>, or <code>~</code>, or that contain any non-alphanumeric characters, ie anything other than <code>A-Z</code> <code>a-z</code> <code>0-9</code> <code>_</code> <code>-</code> <code>~</code>, are not currently supported.</em> Sorry. This may change in the future; <a href="https://github.com/snarfed/bridgy-fed/issues/982">follow this GitHub issue for details</a>.</p>
 <p>Bluesky limits profile bios to 256 characters, so if yours is longer in the fediverse, it will be truncated and ellipsized.</p>
-<p>Alternatively, <a href="#fediverse-follow">you can find and follow bridged Bluesky accounts</a> without bridging your own account, but they won't see your posts or interactions.<p>
+<p>You can also <a href="#fediverse-follow">find and follow bridged Bluesky accounts</a> without bridging your own account, but they won't see your posts or interactions.<p>
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