Help: Document CMAKE_UNITY_BUILD_RELOCATABLE variable

In commit 84996a65aa (Unity: Add option to use relative paths for unity
files, 2024-10-15, v4.0.0-rc1~637^2) we added support for this variable
to initialize the `UNITY_BUILD_RELOCATABLE` target property, but forgot
to document it.

Issue: #26352

Help/manual/cmake-variables.7.rst

@@ -563,6 +563,7 @@ Variables that Control the Build
    /variable/CMAKE_TRY_COMPILE_TARGET_TYPE
    /variable/CMAKE_UNITY_BUILD
    /variable/CMAKE_UNITY_BUILD_BATCH_SIZE
+   /variable/CMAKE_UNITY_BUILD_RELOCATABLE
    /variable/CMAKE_UNITY_BUILD_UNIQUE_ID
    /variable/CMAKE_VERIFY_INTERFACE_HEADER_SETS
    /variable/CMAKE_VISIBILITY_INLINES_HIDDEN

Help/variable/CMAKE_UNITY_BUILD_RELOCATABLE.rst

@@ -0,0 +1,9 @@
+CMAKE_UNITY_BUILD_RELOCATABLE
+-----------------------------
+
+.. versionadded:: 4.0
+
+This variable is used to initialize the :prop_tgt:`UNITY_BUILD_RELOCATABLE`
+property of targets when they are created.  Setting it to true causes
+sources generated for :variable:`CMAKE_UNITY_BUILD` to ``#include`` the
+original source files using relative paths where possible.