added syntax reference for ``ON DUPLICATE KEY`` clause
added syntax reference for ``ON DUPLICATE KEY``
clause

adrpar/crate,crate/crate,EvilMcJerkface/crate,puneetjaiswal/crate,crate/crate,aslanbekirov/crate,gmrodrigues/crate,adrpar/crate,gmrodrigues/crate,gmrodrigues/crate,sunny256/crate,sunny256/crate,puneetjaiswal/crate,husky-koglhof/crate,aslanbekirov/crate,EvilMcJerkface/crate,adrpar/crate,husky-koglhof/crate,crate/crate,puneetjaiswal/crate,EvilMcJerkface/crate,husky-koglhof/crate,aslanbekirov/crate,sunny256/crate