Assorted accessibility (and some consistency) fixes for documentation

Closes #14951 by merging it.

docs/examples/navbar-fixed-top/index.html

@@ -62,7 +62,7 @@
           <ul class="nav navbar-nav navbar-right">
             <li><a href="../navbar/">Default</a></li>
             <li><a href="../navbar-static-top/">Static top</a></li>
-            <li class="active"><a href="./">Fixed top</a></li>
+            <li class="active"><a href="./">Fixed top <span class="sr-only">(current)</span></a></li>
           </ul>
         </div><!--/.nav-collapse -->
       </div>

docs/examples/navbar-static-top/index.html

@@ -61,7 +61,7 @@
           </ul>
           <ul class="nav navbar-nav navbar-right">
             <li><a href="../navbar/">Default</a></li>
-            <li class="active"><a href="./">Static top</a></li>
+            <li class="active"><a href="./">Static top <span class="sr-only">(current)</span></a></li>
             <li><a href="../navbar-fixed-top/">Fixed top</a></li>
           </ul>
         </div><!--/.nav-collapse -->

docs/examples/navbar/index.html

@@ -45,9 +45,9 @@
           </div>
           <div id="navbar" class="navbar-collapse collapse">
             <ul class="nav navbar-nav">
-              <li class="active"><a href="#">Link</a></li>
-              <li><a href="#">Link</a></li>
-              <li><a href="#">Link</a></li>
+              <li class="active"><a href="#">Home</a></li>
+              <li><a href="#">About</a></li>
+              <li><a href="#">Contact</a></li>
               <li class="dropdown">
                 <a href="#" class="dropdown-toggle" data-toggle="dropdown">Dropdown <span class="caret"></span></a>
                 <ul class="dropdown-menu" role="menu">
@@ -62,7 +62,7 @@
               </li>
             </ul>
             <ul class="nav navbar-nav navbar-right">
-              <li class="active"><a href="./">Default</a></li>
+              <li class="active"><a href="./">Default <span class="sr-only">(current)</span></a></li>
               <li><a href="../navbar-static-top/">Static top</a></li>
               <li><a href="../navbar-fixed-top/">Fixed top</a></li>
             </ul>

docs/examples/signin/index.html

@@ -33,8 +33,10 @@
 
       <form class="form-signin" role="form">
         <h2 class="form-signin-heading">Please sign in</h2>
-        <input type="email" class="form-control" placeholder="Email address" required autofocus>
-        <input type="password" class="form-control" placeholder="Password" required>
+        <label for="inputEmail" class="sr-only">Email address</label>
+        <input type="email" id="inputEmail" class="form-control" placeholder="Email address" required autofocus>
+        <label for="inputPassword" class="sr-only">Email address</label>
+        <input type="password" id="inputPassword" class="form-control" placeholder="Password" required>
         <div class="checkbox">
           <label>
             <input type="checkbox" value="remember-me"> Remember me