Document how the SDK is found if SDKROOT is not set

I've intentionally used slightly vague language ("roughly"), as we don't
want to guarantee the exact invocation of `xcrun`, just hint that it's
close to that.

Author: madsmtm

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

@@ -70,4 +70,5 @@ the `-mmacosx-version-min=...`, `-miphoneos-version-min=...` or similar flags
 to disambiguate.
 
 The path to the SDK can be passed to `rustc` using the common `SDKROOT`
-environment variable.
+environment variable, or will be inferred when compiling on host macOS using
+roughly the same logic as `xcrun --sdk macosx --show-sdk-path`.

src/doc/rustc/src/platform-support/apple-ios-macabi.md

@@ -20,7 +20,8 @@ These targets are cross-compiled, and require the corresponding macOS SDK
 iOS-specific headers, as provided by Xcode 11 or higher.
 
 The path to the SDK can be passed to `rustc` using the common `SDKROOT`
-environment variable.
+environment variable, or will be inferred when compiling on host macOS using
+roughly the same logic as `xcrun --sdk macosx --show-sdk-path`.
 
 ### OS version
 

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

@@ -26,7 +26,8 @@ These targets are cross-compiled, and require the corresponding iOS SDK
 ARM64 targets, Xcode 12 or higher is required.
 
 The path to the SDK can be passed to `rustc` using the common `SDKROOT`
-environment variable.
+environment variable, or will be inferred when compiling on host macOS using
+roughly the same logic as `xcrun --sdk iphoneos --show-sdk-path`.
 
 ### OS version
 

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

@@ -20,7 +20,8 @@ These targets are cross-compiled, and require the corresponding tvOS SDK
 ARM64 targets, Xcode 12 or higher is required.
 
 The path to the SDK can be passed to `rustc` using the common `SDKROOT`
-environment variable.
+environment variable, or will be inferred when compiling on host macOS using
+roughly the same logic as `xcrun --sdk appletvos --show-sdk-path`.
 
 ### OS version
 

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

@@ -18,7 +18,8 @@ These targets are cross-compiled, and require the corresponding visionOS SDK
 (`XROS.sdk` or `XRSimulator.sdk`), as provided by Xcode 15 or newer.
 
 The path to the SDK can be passed to `rustc` using the common `SDKROOT`
-environment variable.
+environment variable, or will be inferred when compiling on host macOS using
+roughly the same logic as `xcrun --sdk xros --show-sdk-path`.
 
 ### OS version
 

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

@@ -24,7 +24,8 @@ These targets are cross-compiled, and require the corresponding watchOS SDK
 ARM64 targets, Xcode 12 or higher is required.
 
 The path to the SDK can be passed to `rustc` using the common `SDKROOT`
-environment variable.
+environment variable, or will be inferred when compiling on host macOS using
+roughly the same logic as `xcrun --sdk watchos --show-sdk-path`.
 
 ### OS version