Documentation for smooth lambda option in boundary layer, issue 159

### Are there any linked Issues or Pull Requests?

#159; #451

### Brief description

This new code needs to be added to the Boundary Layer documentation but that is still being ported under #451

### Further details of the issue.

_No response_

Start by reading issues #159 and #451 to understand the smooth lambda option and the Boundary Layer documentation port. Locate the Boundary Layer documentation once #451 establishes its destination, then document the new option and verify that its behavior and configuration are covered.

Written by the indexing model from the issue text.