update armv4t docs

Author: Lokathor

src/doc/rustc/src/platform-support/armv4t-none-eabi.md

@@ -1,16 +1,13 @@
-# armv4t-none-eabi
+# armv4t-none-eabi / thumbv4t-none-eabi
 
 Tier 3
 
-Bare-metal target for any cpu in the Armv4T architecture family, supporting
-ARM/Thumb code interworking (aka `A32`/`T32`), with ARM code as the default code
-generation.
+These two targets are part of the [`arm-none-eabi`](arm-none-eabi.md) target
+group, and all the information there applies.
 
-In particular this supports the Game Boy Advance (GBA), but there's nothing
-GBA-specific with this target, so any Armv4T device should work fine.
-
-See [`arm-none-eabi`](arm-none-eabi.md) for information applicable to all
-`arm-none-eabi` targets.
+Both of these targets can be used on the Game Boy Advance (GBA), among other
+things. On the GBA, one should usually use the `thumb` target to get the best
+overall performance.
 
 ## Target Maintainers
 
@@ -23,6 +20,6 @@ This is a cross-compiled target that you will need to emulate during testing.
 Because this is a device-agnostic target, and the exact emulator that you'll
 need depends on the specific device you want to run your code on.
 
-For example, when programming for the Gameboy Advance, the
-[mgba-test-runner](https://github.com/agbrs/agb) program could be used to make a
-normal set of rust tests be run within the `mgba` emulator.
+* When building for the GBA, [mgba-test-runner](https://github.com/agbrs/agb)
+  can be used to make a normal set of rust tests be run within the `mgba`
+  emulator.