Better document FiniteElement.
Better document FiniteElement.

In particular, document the restriction_is_additive flags for which
there was basically nothing there so far. Also provide better documentation
about the constructor arguments.

johntfoster/dealii,maieneuro/dealii,Arezou-gh/dealii,andreamola/dealii,andreamola/dealii,YongYang86/dealii,sriharisundar/dealii,johntfoster/dealii,sriharisundar/dealii,andreamola/dealii,sriharisundar/dealii,adamkosik/dealii,Arezou-gh/dealii,sairajat/dealii,jperryhouts/dealii,sairajat/dealii,EGP-CIG-REU/dealii,andreamola/dealii,JaeryunYim/dealii,maieneuro/dealii,angelrca/dealii,JaeryunYim/dealii,andreamola/dealii,spco/dealii,adamkosik/dealii,danshapero/dealii,naliboff/dealii,kalj/dealii,sriharisundar/dealii,shakirbsm/dealii,pesser/dealii,maieneuro/dealii,Arezou-gh/dealii,naliboff/dealii,danshapero/dealii,maieneuro/dealii,danshapero/dealii,EGP-CIG-REU/dealii,angelrca/dealii,ESeNonFossiIo/dealii,maieneuro/dealii,ibkim11/dealii,danshapero/dealii,andreamola/dealii,nicolacavallini/dealii,danshapero/dealii,ESeNonFossiIo/dealii,JaeryunYim/dealii,shakirbsm/dealii,EGP-CIG-REU/dealii,YongYang86/dealii,sairajat/dealii,ibkim11/dealii,spco/dealii,Arezou-gh/dealii,YongYang86/dealii,ibkim11/dealii,shakirbsm/dealii,kalj/dealii,pesser/dealii,pesser/dealii,gpitton/dealii,YongYang86/dealii,adamkosik/dealii,angelrca/dealii,jperryhouts/dealii,sriharisundar/dealii,johntfoster/dealii,danshapero/dealii,johntfoster/dealii,ESeNonFossiIo/dealii,ESeNonFossiIo/dealii,johntfoster/dealii,johntfoster/dealii,sairajat/dealii,JaeryunYim/dealii,jperryhouts/dealii,andreamola/dealii,shakirbsm/dealii,kalj/dealii,jperryhouts/dealii,nicolacavallini/dealii,maieneuro/dealii,spco/dealii,pesser/dealii,spco/dealii,ibkim11/dealii,JaeryunYim/dealii,sriharisundar/dealii,shakirbsm/dealii,kalj/dealii,kalj/dealii,kalj/dealii,ESeNonFossiIo/dealii,EGP-CIG-REU/dealii,naliboff/dealii,EGP-CIG-REU/dealii,adamkosik/dealii,gpitton/dealii,naliboff/dealii,adamkosik/dealii,shakirbsm/dealii,ibkim11/dealii,EGP-CIG-REU/dealii,angelrca/dealii,pesser/dealii,naliboff/dealii,jperryhouts/dealii,gpitton/dealii,sairajat/dealii,nicolacavallini/dealii,angelrca/dealii,danshapero/dealii,nicolacavallini/dealii,jperryhouts/dealii,naliboff/dealii,pesser/dealii,ibkim11/dealii,shakirbsm/dealii,jperryhouts/dealii,johntfoster/dealii,YongYang86/dealii,ESeNonFossiIo/dealii,Arezou-gh/dealii,ibkim11/dealii,Arezou-gh/dealii,kalj/dealii,YongYang86/dealii,pesser/dealii,spco/dealii,adamkosik/dealii,sriharisundar/dealii,angelrca/dealii,spco/dealii,nicolacavallini/dealii,gpitton/dealii,JaeryunYim/dealii,maieneuro/dealii,JaeryunYim/dealii,adamkosik/dealii,Arezou-gh/dealii,gpitton/dealii,YongYang86/dealii,naliboff/dealii,nicolacavallini/dealii,gpitton/dealii,nicolacavallini/dealii,spco/dealii,sairajat/dealii,gpitton/dealii,sairajat/dealii,ESeNonFossiIo/dealii,angelrca/dealii,EGP-CIG-REU/dealii