Update search-index.py documentation

Author: mitaa

src/etc/search-index.py

@@ -14,9 +14,11 @@
 search-index.py checks the contents of search-indexes generated by rustdoc
 against a reference, which is being used by compiletest during rustdoc tests.
 
-The reference to check against is embedded into the rustdoc tests source-file,
-fenced by a block comment `/* !search index {} */` so that it can be extracted:
+To add this check to a rustdoc test a `// check-search-index` compiletest directive and
+a reference-index, fencend in `/* !search-index ... */`, needs to be inserted:
 ```
+// check-search-index
+
 pub struct foo {
     field: i32,
 }
@@ -32,6 +34,14 @@
 */
 ```
 
+The reference-index can be written by hand or dumped from an existing search-index,
+requiring the rustdoc test to be built by rustdoc beforehand, through:
+`python search-index.py dump .../search-index.js`
+
+which can then be copied directly into the rustdoc test.
+
+
+
 The reference-index, a simplified json-format of the search-index, is structured like this:
 ```
 {