doc: rename HTML element IDs to avoid duplicates

These 3 release notes have had an element ID collision because both
the runtime changes and changes to the package "runtime" used the
same ID. Fix it by using a "pkg-" prefix for the runtime package.

Move the "runtime-again" ID from CL 129635 to a nearby <dt> element
so that existing links to https://golang.org/doc/go1.11#runtime-again
don't break.

Fixes #37036
Updates #36878

Change-Id: Ib68d93acfac802fd84c0a57485937e45dea2064a
Reviewed-on: https://go-review.googlesource.com/c/go/+/217797
Reviewed-by: Toshihiro Shiino <shiino.toshihiro@gmail.com>
Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.11.html

@@ -826,7 +826,7 @@ for k := range m {
 
 <!-- CL 101715 was reverted -->
 
-<dl id="runtime-again"><dt><a href="/pkg/runtime/">runtime</a></dt>
+<dl id="pkg-runtime"><dt id="runtime-again"><a href="/pkg/runtime/">runtime</a></dt>
   <dd>
 
     <p><!-- CL 70993 -->

doc/go1.13.html

@@ -904,7 +904,7 @@ godoc
 
 </dl><!-- reflect -->
 
-<dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
+<dl id="pkg-runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
   <dd>
     <p> <!-- CL 167780 -->
       Tracebacks, <a href="/pkg/runtime/#Caller"><code>runtime.Caller</code></a>,

doc/go1.14.html

@@ -736,7 +736,7 @@ appropriately.)
 
 </dl><!-- reflect -->
 
-<dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
+<dl id="pkg-runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
   <dd>
     <p><!-- CL 200081 -->
       <code>runtime.Goexit</code> can no longer be aborted by a

doc/go1.9.html

@@ -862,7 +862,7 @@ version of gccgo.
 
 </dl><!-- reflect -->
 
-<dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
+<dl id="pkg-runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
   <dd>
     <p><!-- CL 37233, CL 37726 -->
       Tracebacks generated by the runtime and recorded in profiles are