rustdoc-search: show type signature on type-driven SERP

This displays the function signature, as rustdoc understands it,
on the In Parameters, In Return Types, and In Function Signature
pages, but not in the In Names page, since it's not used there.
It also highlights the matching parts, to clarify why a function
is considered a good match.

src/librustdoc/html/static/css/rustdoc.css

@@ -328,10 +328,11 @@ details:not(.toggle) summary {
 	margin-bottom: .6em;
 }
 
-code, pre, a.test-arrow, .code-header {
+code, pre, a.test-arrow, .code-header, .search-results .type-signature {
 	font-family: "Source Code Pro", monospace;
 }
-.docblock code, .docblock-short code {
+.docblock code, .docblock-short code,
+.search-results .type-signature strong {
 	border-radius: 3px;
 	padding: 0 0.125em;
 }
@@ -681,7 +682,8 @@ ul.block, .block li {
 }
 
 .docblock code, .docblock-short code,
-pre, .rustdoc.src .example-wrap {
+pre, .rustdoc.src .example-wrap,
+.search-results .type-signature strong {
 	background-color: var(--code-block-background-color);
 }