Specutils needs to document how to create a Spectrum1D using flambda units

It would be very helpful if the specutils documentation would show how to create a Spectrum1D using flambda (specific intensity, for example erg/s/cm^2/Angstrom) as the flux unit. The only examples in the documentation at present are for fnu (flux density, for example, Jy).

Both fnu and flambda are used widely in astronomy. Confusing fnu for flambda is one of the most common errors in astronomy, which could be prevented by better documentation. And since there is no shortcut unit when working with flambda, it's not at all obvious how to input an flambda spectrum into Spectrum1D. This creates a big barrier to entry for new users.

Start by locating the existing documentation examples for creating a Spectrum1D with fnu units and compare their structure with the requested flambda case. Done means the documentation shows a clear flambda example using specific intensity units such as erg/s/cm^2/Angstrom and explains how it differs from fnu.

Written by the indexing model from the issue text.