arm64: documentation: tighten up tagged pointer documentation

Commit d50240a5 ("arm64: mm: permit use of tagged pointers at EL0")
added support for tagged pointers in userspace, but the corresponding
update to Documentation/ contained some imprecise statements.

This patch fixes up some minor ambiguities in the text, hopefully making
it more clear about exactly what the kernel expects from user virtual
addresses.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>