Clarify that SiteInstance::GetSiteURL is usually not accurate.

content/public/browser/site_instance.h

Index: content/public/browser/site_instance.h
diff --git a/content/public/browser/site_instance.h b/content/public/browser/site_instance.h
index 1ab1b5c3c03bab48a4aea0b6c69b9dd17d885f98..708da824c2951d4ccfd64de1e7c9410b77fe9a98 100644
--- a/content/public/browser/site_instance.h
+++ b/content/public/browser/site_instance.h
@@ -43,7 +43,8 @@ class RenderProcessHost;
 // (which uses the same BrowsingInstance).  If the user navigates within a site,
 // the same SiteInstance is used.  Caveat: we currently allow renderer-initiated
 // cross-site navigations to stay in the same SiteInstance, to preserve
-// compatibility in cases like cross-site iframes that open popups.
+// compatibility in cases like cross-site iframes that open popups.  This means
+// that most SiteInstances will contain pages from multiple sites.
 //
 // SITE PER PROCESS (currently experimental): is the most granular process
 // model and is made possible by our support for out-of-process iframes.  A
@@ -61,7 +62,9 @@ class RenderProcessHost;
 //
 // PROCESS PER SITE: We consolidate all SiteInstances for a given site into the
 // same process, throughout the entire browser context.  This ensures that only
-// one process will be used for each site.
+// one process will be used for each site.  Note that there is no strict process
+// isolation of sites in this mode, so a given SiteInstance can still contain
+// pages from multiple sites.
 //
 // Each NavigationEntry for a WebContents points to the SiteInstance that
 // rendered it.  Each RenderFrameHost also points to the SiteInstance that it is
@@ -84,12 +87,12 @@ class CONTENT_EXPORT SiteInstance : public base::RefCounted<SiteInstance> {
   // Returns the current RenderProcessHost being used to render pages for this
   // SiteInstance.  If there is no RenderProcessHost (because either none has
   // yet been created or there was one but it was cleanly destroyed (e.g. when
-  // it is not actively being used)), then this method will create a new
+  // it is not actively being used), then this method will create a new
   // RenderProcessHost (and a new ID).  Note that renderer process crashes leave
   // the current RenderProcessHost (and ID) in place.
   //
   // For sites that require process-per-site mode (e.g., WebUI), this will
-  // ensure only one RenderProcessHost for the site exists/ within the
+  // ensure only one RenderProcessHost for the site exists within the
   // BrowserContext.
   virtual content::RenderProcessHost* GetProcess() = 0;
 
@@ -97,8 +100,14 @@ class CONTENT_EXPORT SiteInstance : public base::RefCounted<SiteInstance> {
   // SiteInstances) belongs.
   virtual content::BrowserContext* GetBrowserContext() const = 0;
 
-  // Get the web site that this SiteInstance is rendering pages for.
-  // This includes the scheme and registered domain, but not the port.
+  // Get the web site that this SiteInstance is rendering pages for.  This
+  // includes the scheme and registered domain, but not the port.
+  //
+  // NOTE: In most cases, this value should not be considered authoritative
+  // because a SiteInstance can usually host pages from multiple sites.  It is
+  // only an accurate representation of the pages within the SiteInstance in
+  // the "site per process" process model, or for sites that require process
+  // isolation (e.g., WebUI, extensions).
   virtual const GURL& GetSiteURL() const = 0;
 
   // Gets a SiteInstance for the given URL that shares the current