Help Me Remember: A Developer Reference App
It sounds like you're talking about Dash. It has a global search keyboard shortcut feature as well as a host of others for searching developer documentation.
Here's an overview of the app from its website:

Dash is an API Documentation Browser and Code Snippet Manager. Dash instantly searches offline documentation sets for 200+ APIs, 100+ cheat sheets and more.

And a summary of the global shortcut feature:

The global search shortcut is an easy way to activate Dash in a similar style as the Spotlight shortcut. You can assign a search shortcut in Preferences &gt; General and then use that hotkey in any app when you want to bring up Dash. You can also assign a shortcut to search using the selected text.