[llvm][docs] MemTagSanitizer is only supported on AArch64 Android (#120545)

```
$ ./bin/clang /tmp/test.c -o /tmp/test.o -target aarch64-linux -march=armv8+memtag -fsanitize=memtag-stack
clang: error: unsupported option '-fsanitize=memtag*' for target 'aarch64-unknown-linux'
```
But this works:
```
$ ./bin/clang /tmp/test.c -o /tmp/test.o --target=aarch64-linux-android -march=armv8+memtag -fsanitize=memtag-stack
```

Due to this check in Clang:

https://github.com/llvm/llvm-project/blob/2210da3b823ccf21fc634c858827c9f12c864b51/clang/lib/Driver/ToolChains/CommonArgs.cpp#L1651

Likely because the required notes and dynamic loader support only exist
for Android.

You can get around this, sort of, by not linking the file. However this
means you have to provide your own way of loading it, so it doesn't
change the statement that this feature is Android only.

#64692 also confirms that the
intent is to only support Android at this time.

And while I'm here, suggest an additive set of flags that can also be
used.

Author: DavidSpickett

llvm/docs/MemTagSanitizer.rst

@@ -28,10 +28,15 @@ memory bugs.
 Usage
 =====
 
-Compile and link your program with ``-fsanitize=memtag`` flag. This
-will only work when targeting AArch64 with MemTag extension. One
-possible way to achieve that is to add ``-target
-aarch64-linux -march=armv8+memtag`` to compilation flags.
+Compile and link your program with the ``-fsanitize=memtag`` flag. This
+will only work when targeting AArch64 Android with the memory tagging extension.
+One possible way to achieve that is to add ``--target=aarch64-linux-android -march=armv8+memtag``
+to your compilation flags.
+
+Note that doing this will override existing flags of the same type. Assuming that
+you are already targeting AArch64 Android, an alternative is to add
+``-Xclang -target-feature -Xclang +mte`` to your compilation flags. This
+adds the memory tagging feature, without changing anything else.
 
 Implementation
 ==============