Document default boundary condition for Stokes solve

[naliboff]

Per this post on the forum, it is not clear from the manual that if one does not specify a boundary conditions (velocity or traction) for a given boundary, it will be assigned a pressure boundary condition. This should be documented in the manual, unless my quick search simply did not find where this is specified.

@danieldouglas92 @PrajaktaPMohite @flacombe1 - A good project to tackle!

[bangerth]

I assume you mean "...it will be assigned a traction boundary condition". You can impose tractions, but not pressures.