Update to r42905:

• adds pkg instructions to 6.3. Installing Xorg
• adds a Caveats section to 6.4. Xorg Configuration
• temporarily moves the extra config stuff to a new section called 6.8. Troubleshooting
• replaces instances of X and X11 with the entity for xorg

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D3964