add REST documentation for search module

Can you add the other ones that are registered as search types here as well? I know it's against the grain on separation of concerns, but it's a pretty loose reference and helpful for those reading the docs.

```
~/Source/oaeproject/Hilary$ git grep "registerSearch("
node_modules/oae-following/lib/search.js:SearchAPI.registerSearch('followers', require('./search/searches/followers'));
node_modules/oae-following/lib/search.js:SearchAPI.registerSearch('following', require('./search/searches/following'));
node_modules/oae-library/lib/api.search.js:    SearchAPI.registerSearch(searchName, function(ctx, opts, callback) {
node_modules/oae-principals/lib/search.js:SearchAPI.registerSearch('members', require('./search/searches/members'));
node_modules/oae-search/lib/init.js:    SearchAPI.registerSearch('general', require('./searches/general'));
node_modules/oae-search/tests/test-search-api.js:        SearchAPI.registerSearch('test-registerSearch', function() { });
node_modules/oae-search/tests/test-search-api.js:            SearchAPI.registerSearch('test-registerSearch', function() { });
```