Create docs_url and summary attributes in class HostingService and SCMTools. 
Fill details in each hosting services (not include Kiln On Demand and Review Board Gateway, because right now we don't have references provided by ReviewBoard Manual. 


Hosting Servive: $hostingHelp is a new paragraph with class="hosting-help-text" in the template for the RepositoryForm javascript file. By default, $hostingHelp will nonvisiable. When users select a hosting service, the inline text will show up.


SCMTools: $repoHelp is a new paragraph with class="repo-help-text" in the template for the RepositoryForm javascript file. Only when a user click 'custom' as the hosting service, $repoHelp will visiable. When users select a repo type, the inline text will show up.

Tested in Google Chrome, Firefox and Safari. Inline Text of each hosting service displays correctly in the page.

Tested ./tests/runtests.py reviewboard.webapi.tests.test_hosting_service successfully.