arm64: documentation: tighten up tagged pointer documentation

Commit d50240a ("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>

Documentation/arm64/tagged-pointers.txt

@@ -18,17 +18,17 @@ this byte for application use, with the following caveats:
 	    parameters containing user virtual addresses *must* have
 	    their top byte cleared before trapping to the kernel.
 
-	(2) Tags are not guaranteed to be preserved when delivering
-	    signals. This means that signal handlers in applications
-	    making use of tags cannot rely on the tag information for
-	    user virtual addresses being maintained for fields inside
-	    siginfo_t. One exception to this rule is for signals raised
-	    in response to debug exceptions, where the tag information
+	(2) Non-zero tags are not preserved when delivering signals.
+	    This means that signal handlers in applications making use
+	    of tags cannot rely on the tag information for user virtual
+	    addresses being maintained for fields inside siginfo_t.
+	    One exception to this rule is for signals raised in response
+	    to watchpoint debug exceptions, where the tag information
 	    will be preserved.
 
 	(3) Special care should be taken when using tagged pointers,
 	    since it is likely that C compilers will not hazard two
-	    addresses differing only in the upper bits.
+	    virtual addresses differing only in the upper byte.
 
 The architecture prevents the use of a tagged PC, so the upper byte will
 be set to a sign-extension of bit 55 on exception return.