Fix reset to BL3-1 instructions in user guide, part 2

Fix the instructions for resetting to the BL3-1 entrypoint in the
user guide. The BL3-1 and BL3-2 image locations changed in the fix
to ARM-software/tf-issues#100 (commit 186c1d4). This is distinct
from the similar issue fixed in commit bfb1dd5.

Also clarify the dependence on the FVP_SHARED_DATA_LOCATION and
FVP_TSP_RAM_LOCATION build options, and tidy up the "Notes
regarding Base FVP configuration options" section.

Change-Id: I6b03452a71f0c69efa169852712bcb184242696e