Clarify that only lowercase letters are allowed

We only allow lowercase letters in localparts.

Fixes element-hq/element-web#9983

src/components/views/auth/RegistrationForm.js

@@ -380,7 +380,7 @@ module.exports = React.createClass({
     },
 
     validateUsernameRules: withValidation({
-        description: () => _t("Use letters, numbers, dashes and underscores only"),
+        description: () => _t("Use lowercase letters, numbers, dashes and underscores only"),
         rules: [
             {
                 key: "required",

src/i18n/strings/en_EN.json

@@ -304,7 +304,6 @@
     "Custom user status messages": "Custom user status messages",
     "Group & filter rooms by custom tags (refresh to apply changes)": "Group & filter rooms by custom tags (refresh to apply changes)",
     "Render simple counters in room header": "Render simple counters in room header",
-    "Custom Notification Sounds": "Custom Notification Sounds",
     "Edit messages after they have been sent (refresh to apply changes)": "Edit messages after they have been sent (refresh to apply changes)",
     "React to messages with emoji (refresh to apply changes)": "React to messages with emoji (refresh to apply changes)",
     "Enable Emoji suggestions while typing": "Enable Emoji suggestions while typing",
@@ -1356,7 +1355,7 @@
     "Other users can invite you to rooms using your contact details": "Other users can invite you to rooms using your contact details",
     "Enter phone number (required on this homeserver)": "Enter phone number (required on this homeserver)",
     "Doesn't look like a valid phone number": "Doesn't look like a valid phone number",
-    "Use letters, numbers, dashes and underscores only": "Use letters, numbers, dashes and underscores only",
+    "Use lowercase letters, numbers, dashes and underscores only": "Use lowercase letters, numbers, dashes and underscores only",
     "Enter username": "Enter username",
     "Some characters not allowed": "Some characters not allowed",
     "Email (optional)": "Email (optional)",