docs: add some notes and warnings about cross-compilation (#985)

See #977.

---------

Signed-off-by: Henry Schreiner <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: henryiii

docs/build.md

@@ -16,7 +16,7 @@ pipx run build
 ````{tab} uv
 
 ```bash
-uvx --from build pyproject-build --installer=uv
+uv build
 ```
 
 ````

docs/crosscompile.md

@@ -2,12 +2,25 @@
 
 ## macOS
 
+Unlike the other platforms, macOS has the ability to target older operating
+systems with the `MACOSX_DEPLOYMENT_TARGET` variable. If that is not set, you
+will get a wheel optimized for the current operating system. Popular
+redistributable builders like cibuildwheel will set this for you.
+
+:::{warning}
+
+While CMake also allows you to specify this a few other ways, scikit-build-core
+will not know you've set this and won't get the correct wheel name.
+
+:::
+
 ### Intel to AppleSilicon
 
 On macOS, AppleClang has excellent support for making Apple Silicon and
 Universal2 binaries (both architectures in one binary). Scikit-build-core
 respects `ARCHFLAGS` if `CMAKE_SYSTEM_PROCESSOR` is not in the cmake args. These
-values are set by cibuildwheel when cross-compiling.
+values are set by most redistributable builders like cibuildwheel when
+cross-compiling.
 
 :::{warning}
 
@@ -19,6 +32,13 @@ Universal2.
 
 :::
 
+:::{warning}
+
+If you manually set the arch flags in other ways besides `ARCHFLAGS`, or the one
+special case above, scikit-build-core will not get the right wheel name.
+
+:::
+
 ## Windows
 
 ### Intel to ARM