Rollup merge of #78103 - GuillaumeGomez:rustdoc-book, r=jyn514

Add link to rustdoc book in rustdoc help popup

Part of #75520.

It looks like this:

![Screenshot from 2020-10-19 13-46-02](https://user-images.githubusercontent.com/3050060/96446334-934d6900-1211-11eb-8fdc-133fecc8c30d.png)
![Screenshot from 2020-10-19 13-43-46](https://user-images.githubusercontent.com/3050060/96446335-947e9600-1211-11eb-955c-68af5292aecc.png)
![Screenshot from 2020-10-19 13-37-26](https://user-images.githubusercontent.com/3050060/96446337-947e9600-1211-11eb-9a2e-399b99178a65.png)

r? @jyn514

src/librustdoc/html/static/main.js

@@ -2792,6 +2792,10 @@ function defocusSearchBar() {
         addClass(popup, "hidden");
         popup.id = "help";
 
+        var book_info = document.createElement("span");
+        book_info.innerHTML = "You can find more information in \
+            <a href=\"https://doc.rust-lang.org/rustdoc/\">the rustdoc book</a>.";
+
         var container = document.createElement("div");
         var shortcuts = [
             ["?", "Show this help dialog"],
@@ -2825,6 +2829,7 @@ function defocusSearchBar() {
         addClass(div_infos, "infos");
         div_infos.innerHTML = "<h2>Search Tricks</h2>" + infos;
 
+        container.appendChild(book_info);
         container.appendChild(div_shortcuts);
         container.appendChild(div_infos);
 

src/librustdoc/html/static/rustdoc.css

@@ -796,14 +796,22 @@ body.blur > :not(#help) {
 	clear: left;
 	display: block;
 }
+#help > div > span {
+	text-align: center;
+	display: block;
+	margin: 10px 0;
+	font-size: 18px;
+	border-bottom: 1px solid #ccc;
+	padding-bottom: 4px;
+	margin-bottom: 6px;
+}
 #help dd { margin: 5px 35px; }
 #help .infos { padding-left: 0; }
 #help h1, #help h2 { margin-top: 0; }
 #help > div div {
 	width: 50%;
 	float: left;
-	padding: 20px;
+	padding: 0 20px 20px 17px;;
-	padding-left: 17px;
 }
 
 .stab {

src/librustdoc/html/static/themes/ayu.css

@@ -219,7 +219,8 @@ a {
 }
 
 .docblock:not(.type-decl) a:not(.srclink):not(.test-arrow),
-.docblock-short a:not(.srclink):not(.test-arrow), .stability a {
+.docblock-short a:not(.srclink):not(.test-arrow), .stability a,
+#help a {
 	color: #39AFD7;
 }
 
@@ -275,6 +276,10 @@ a {
 	border-radius: 4px;
 }
 
+#help > div > span {
+	border-bottom-color: #5c6773;
+}
+
 .since {
 	color: grey;
 }

src/librustdoc/html/static/themes/dark.css

@@ -177,7 +177,8 @@ a {
 }
 
 .docblock:not(.type-decl) a:not(.srclink):not(.test-arrow),
-.docblock-short a:not(.srclink):not(.test-arrow), .stability a {
+.docblock-short a:not(.srclink):not(.test-arrow), .stability a,
+#help a {
 	color: #D2991D;
 }
 
@@ -231,6 +232,10 @@ a.test-arrow {
 	border-color: #bfbfbf;
 }
 
+#help > div > span {
+	border-bottom-color: #bfbfbf;
+}
+
 #help dt {
 	border-color: #bfbfbf;
 	background: rgba(0,0,0,0);

src/librustdoc/html/static/themes/light.css

@@ -175,7 +175,8 @@ a {
 }
 
 .docblock:not(.type-decl) a:not(.srclink):not(.test-arrow),
-.docblock-short a:not(.srclink):not(.test-arrow), .stability a {
+.docblock-short a:not(.srclink):not(.test-arrow), .stability a,
+#help a {
 	color: #3873AD;
 }
 
@@ -229,6 +230,10 @@ a.test-arrow {
 	border-color: #bfbfbf;
 }
 
+#help > div > span {
+	border-bottom-color: #bfbfbf;
+}
+
 .since {
 	color: grey;
 }