Given synonym of "administration password" as people use them (#15571)

To improve SEO

wiki/src/doc/first_steps/startup_options/administration_password.mdwn

 [[!meta title="Administration password"]]
 
-In Tails, an administration password is required to perform system
-administration tasks.<br/>
+In Tails, an administration password (also called *root password* or *amnesia password*) is required to perform system
+administration tasks.
 For example:
 
   - To [[install additional software|doc/advanced_topics/additional_software]]
@@ -13,7 +13,7 @@ This can prevent an attacker with physical or remote access to your Tails system
 to gain administration privileges and perform administration tasks
 against your will.
 
-[[!img password-prompt.png link="no" alt="Authentication required: amnesia password"]]
+[[!img password-prompt.png link="no" alt="Authentication required: amnesia password (also called *administration password* or *root password*)"]] <!-- Note for translators: the alt tag is useful for SEO. -->
 
 Set up an administration password
 =================================