Expand cross-building.md with general definition, sysroot explanation, and arm64 example

Co-authored-by: mthalman <15789599+mthalman@users.noreply.github.com>

Author: Copilot

Documentation/cross-building.md

@@ -1,27 +1,36 @@
 # Cross-Building
 
-Cross-building is the process of building .NET for a target platform on a
-different host platform. For example, building for `linux-x64` on a host that
-uses the `azurelinux-3.0-net10.0-cross-amd64` container image with a
-dedicated sysroot.
+Cross-building is the process of building .NET on a host machine for a target
+OS and/or CPU architecture that differs from the host. For example, compiling
+`linux-arm64` binaries on an `x64` host, or building for a specific Linux
+distribution using a container image that provides a dedicated sysroot.
+
+A sysroot is a self-contained root filesystem for the target platform. It
+contains the headers, libraries, and other files needed to compile and link
+code for that target. Cross-build container images provided by the .NET team
+(see [dotnet-buildtools-prereqs-docker](https://github.com/dotnet/dotnet-buildtools-prereqs-docker))
+typically place the sysroot at a well-known path and expose it via the
+`ROOTFS_DIR` environment variable.
 
 ## When is Cross-Building Required?
 
-Cross-building is required when:
+Cross-building is required when any of the following are true:
 
-- The build environment uses a cross-build container image (e.g.
-  `mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-net10.0-cross-amd64`)
-  that provides a sysroot via the `ROOTFS_DIR` environment variable, even when
-  building for the same architecture as the host (e.g. `linux-x64`).
+- The **target CPU architecture** differs from the host (e.g. building
+  `linux-arm64` on an `x64` machine).
+- The **target OS or Linux distribution** requires a dedicated sysroot that is
+  provided by a cross-build container image via the `ROOTFS_DIR` environment
+  variable, even when the host and target share the same CPU architecture
+  (e.g. building `linux-x64` inside an
+  `azurelinux-3.0-net10.0-cross-amd64` container).
 
 ## Enabling Cross-Building
 
-When building in a cross-build container with `ROOTFS_DIR` set, you must pass
-`/p:CrossBuild=true` to the build command. Without this flag, the runtime build
-will not use the sysroot when searching for native dependencies such as OpenSSL,
-causing the build to fail.
+When `ROOTFS_DIR` is set, you must pass `/p:CrossBuild=true` to the build
+command. Without this flag, the runtime build will not use the sysroot when
+searching for native dependencies such as OpenSSL, causing the build to fail.
 
-## Example
+## Example: Building linux-x64 in a Cross-Build Container
 
 ```bash
 docker run \
@@ -38,6 +47,22 @@ docker run \
     /p:CrossBuild=true
 ```
 
-> **Note:** `/p:CrossBuild=true` is only required when using a cross-build
-> container with `ROOTFS_DIR` set. It is not needed when building natively on
-> the target platform.
+## Example: Building linux-arm64 on an x64 Host
+
+```bash
+docker run \
+  --rm \
+  --volume /path/to/dotnet:/dotnet \
+  --workdir /dotnet \
+  --env ROOTFS_DIR=/crossrootfs/arm64 \
+  mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-net10.0-cross-arm64 \
+  ./build.sh \
+    --clean-while-building \
+    --source-only \
+    --arch arm64 \
+    --os linux \
+    /p:CrossBuild=true
+```
+
+> **Note:** `/p:CrossBuild=true` is only required when `ROOTFS_DIR` is set.
+> It is not needed when building natively on the target platform.