arm64/mte: Clarify mode reported by PR_GET_TAGGED_ADDR_CTRL

With the current wording readers might infer that PR_GET_TAGGED_ADDR_CTRL
will report the mode currently active in the thread however this is not the
actual behaviour, instead all modes currently selected by the process will
be reported with the mode used depending on the combination of the
requested modes and the default set for the current CPU. This has been the
case since 433c38f4 ("arm64: mte: change ASYNC and SYNC TCF settings
into bitfields"), before that we did not allow more than one mode to be
requested simultaneously.

Update the documentation to more clearly reflect current behaviour.
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220127190324.660405-1-broonie@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

Documentation/arm64/memory-tagging-extension.rst

@@ -91,8 +91,9 @@ mode is specified, the program will run in that mode. If multiple
 modes are specified, the mode is selected as described in the "Per-CPU
 preferred tag checking modes" section below.
 
-The current tag check fault mode can be read using the
-``prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)`` system call.
+The current tag check fault configuration can be read using the
+``prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)`` system call. If
+multiple modes were requested then all will be reported.
 
 Tag checking can also be disabled for a user thread by setting the
 ``PSTATE.TCO`` bit with ``MSR TCO, #1``.