docs: be explicit about what it means to "use SKBUILD_SOABI" (#646)

I'm not super experienced with building Python modules with cmake, so it
wasn't clear to me until I found it in a test what exactly "use
SKBUILD_SOABI" meant until I found it in one of the tests, so I thought
others might like to see the `set(Python_SOABI ${SKBUILD_SOABI})`.

---------

Signed-off-by: Henry Schreiner <[email protected]>
Co-authored-by: Henry Schreiner <[email protected]>

Author: minrk

docs/cmakelists.md

@@ -42,7 +42,26 @@ If you are making a Limited ABI / Stable API package, you'll need the
 
 If you want to cross-compile to Windows ARM, you'll need to use
 `${SKBUILD_SOABI}`, which is always correct, instead of trusting FindPython's
-`Python_SOABI` value.
+`Python_SOABI` value. You can manually set the extension suffix after making a
+target:
+
+```cmake
+if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
+    set_property (TARGET ${name} PROPERTY SUFFIX "$.{SKBUILD_SOSABI}.pyd")
+else()
+    set_property (TARGET ${name} PROPERTY SUFFIX ".${SKBUILD_SOSABI}${CMAKE_SHARED_MODULE_SUFFIX}")
+endif()
+```
+
+A quicker way to do this would be to instead override `Python_SOABI` after
+`find_package(Python)`:
+
+```cmake
+set(Python_SOABI ${SKBUILD_SOABI})
+```
+
+However, this isn't officially supported upstream, and only works due to the way
+this variable is used when creating targets.
 
 :::