StreamGrepTool is not documented

There is no detail at [https://github.com/gchq/stroom-docs/tree/master/user-guide/tools](https://github.com/gchq/stroom-docs/tree/master/user-guide/tools)

Start by inspecting the user-guide/tools directory linked in the issue and searching the documentation repository for references to StreamGrepTool. Add the missing user-facing detail in the appropriate tools documentation location, then verify that the tool has a clear documentation entry.

Written by the indexing model from the issue text.