Feature/interface doc fix

* Documentation PR to reflect the changes in PR https://github.com/caskdata/tigon/pull/87
Currently, TigonSQL creates an interface set for every input that is added, using the same name as the input for the set. When an input is referenced in a TigonSQL query, the user needs to use the form
\`\`[\<inputName>].inputName\`\`. 

In the example below, \`\`intInput\`\` is both the input name and the interface set; in the query,
we refer to this source as \`\`[intInput].intInput\`\`. Interface sets are currently \*\*not\*\* exposed to TigonSQL users. This may change in a future release to enable more powerful queries.

Though it is not necessary to know about them to use TigonSQL, you can refer to
the :doc:\`Tigon SQL User Manual \</apis/index>\` for more information about interface sets.

\[this URL is probably going to change in the future when we bump the version, so I'd prefer to not directly link to the doc.]