make it more clear what 'Tier 2' (without host tools) means

Author: RalfJung

src/doc/rustc/src/platform-support.md

@@ -63,8 +63,9 @@ Tools](#tier-1-with-host-tools).
 ## Tier 2 with Host Tools
 
 Tier 2 targets can be thought of as "guaranteed to build". The Rust project
-builds official binary releases for each tier 2 target, and automated builds
-ensure that each tier 2 target builds after each change. Automated tests are
+builds official binary releases of the standard library (or, in some cases,
+only the `core` library) for each tier 2 target, and automated builds
+ensure that each tier 2 target can be used as build target after each change. Automated tests are
 not always run so it's not guaranteed to produce a working build, but tier 2
 targets often work to quite a good degree and patches are always welcome!
 
@@ -103,11 +104,12 @@ target | notes
 `x86_64-unknown-linux-musl` | 64-bit Linux with MUSL
 [`x86_64-unknown-netbsd`](platform-support/netbsd.md) | NetBSD/amd64
 
-## Tier 2
+## Tier 2 without Host Tools
 
 Tier 2 targets can be thought of as "guaranteed to build". The Rust project
-builds official binary releases for each tier 2 target, and automated builds
-ensure that each tier 2 target builds after each change. Automated tests are
+builds official binary releases of the standard library (or, in some cases,
+only the `core` library) for each tier 2 target, and automated builds
+ensure that each tier 2 target can be used as build target after each change. Automated tests are
 not always run so it's not guaranteed to produce a working build, but tier 2
 targets often work to quite a good degree and patches are always welcome! For
 the full requirements, see [Tier 2 target