Help: Update Sphinx versionadded directives for 3.20 release

Run the script:

    Utilities/Sphinx/update_versions.py --since v3.19.0 --overwrite

Manually restore the 3.20 version for `cmake_path`, which was originally
part of 3.19 but reverted and restored in 3.20.

Help/policy/CMP0118.rst

@@ -1,6 +1,8 @@
 CMP0118
 -------
 
+.. versionadded:: 3.20
+
 The :prop_sf:`GENERATED` source file property is now visible in all directories.
 
 Whether or not a source file is generated is an all-or-nothing global

Help/prop_tgt/UNITY_BUILD_UNIQUE_ID.rst

@@ -1,6 +1,8 @@
 UNITY_BUILD_UNIQUE_ID
 ---------------------
 
+.. versionadded:: 3.20
+
 The name of a valid C-identifier which is set to a unique per-file
 value during unity builds.
 

Help/variable/CMAKE_UNITY_BUILD_UNIQUE_ID.rst

@@ -1,6 +1,8 @@
 CMAKE_UNITY_BUILD_UNIQUE_ID
 ---------------------------
 
+.. versionadded:: 3.20
+
 This variable is used to initialize the :prop_tgt:`UNITY_BUILD_UNIQUE_ID`
 property of targets when they are created.  It specifies the name of the
 unique identifier generated per file in a unity build.