Update platforms.md documentation

Closes #8513.

PiperOrigin-RevId: 255944103

site/docs/platforms.md

@@ -99,28 +99,17 @@ Note that it is an error for a platform to specify more than one value of the
 same constraint setting, such as `@platforms//cpu:x86_64` and
 `@platforms//cpu:arm` for `@platforms//cpu:cpu`.
 
-## Built-in constraints and platforms
 
-Bazel ships with constraint definitions for the most popular CPU architectures
-and operating systems. These are all located in the repository
-`@platforms`. This repository is developed at
-[github.com/bazelbuild/platforms](https://github.com/bazelbuild/platforms):
+## Generally useful constraints and platforms
 
-*  `//cpu:cpu` for the CPU architecture, with values `//cpu:x86_32`,
-   `//cpu:x86_64`, `//cpu:ppc`, `//cpu:arm`, `//cpu:s390x`
-*  `//os:os` for the operating system, with values `//os:android`,
-   `//os:freebsd`, `//os:ios`, `//os:linux`, `//os:osx`, `//os:windows`
+To keep the ecosystem consistent, Bazel team maintains a repository with
+constraint definitions for the most popular CPU architectures and operating
+systems. These are all located in
+[https://github.com/bazelbuild/platforms](https://github.com/bazelbuild/platforms).
 
-There are also the following special platform definitions:
-
-*  `@bazel_tools//platforms:host_platform` - represents the CPU and operating
-   system for the host environment
-
-*  `@bazel_tools//platforms:target_platform` - represents the CPU and operating
-   system for the target environment
-
-The CPU values used by these two platforms can be specified with the
-`--host_cpu` and `--cpu` flags.
+Bazel ships with the following special platform definition:
+`@local_config_platform//:host`. This is the autodetected host platform value -
+represents autodetected platform for the system Bazel is running on.
 
 ## Specifying a platform for a build