Recomment utf8mb4 for MySQL >= 5.7.7 only

It has innodb_large_prefix by default which allows to have loger indexes
than 768 bytes (what translates to 191 chars only with utf8mb4).

Fixes #938
Signed-off-by: Michal Čihař <michal@cihar.com>

docs/admin/install.rst

@@ -251,9 +251,9 @@ When using MySQL, don't forget to create database with UTF-8 encoding:
     # Grant all privileges to  weblate user
     GRANT ALL PRIVILEGES ON weblate.* TO 'weblate'@'localhost'  IDENTIFIED BY 'password';
 
-    # Create database
+    # Create database on MySQL >= 5.7.7
     CREATE DATABASE weblate CHARACTER SET utf8mb4;
-    # Use utf8 if above fails:
+    # Use utf8 for older versions
     # CREATE DATABASE weblate CHARACTER SET utf8;
 
 Other configurations