add REST documentation for search module

Grepping for the registration pattern (as per @mrvisser suggestion in earlier comment) was how I got the four original search types.

```
$ 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', functi
```

It took a search of the test code to find `content-library` and `memberships-library`.

Maybe those six types are all that exist currently? It does seem, though, that this will be quite a problem to keep up-to-date as new search types are added. Would it be better to define this parameter as a simple string (without explicit enumeration) and then simply list the "known" search types in the description as examples?