[docs] Adjust variable formatting table

While the in-place hints on valid formats are up to date (e.g. when
choosing an invalid format expr -f nonExisting -- 42), the corresponding
online docs table is not. The formats "address", "hex float",
"instruction" and "void" are missing, and "decimal" refers to an
outdated abbreviation 'i' instead of 'd'.

Patch by: Lukas Böger

Differential revision: https://reviews.llvm.org/D63813

llvm-svn: 366364