Help: Document when HIP_COMPILER generator expressions were added

These were added by commit b50bfc8913 (HIP: Add language to CMake,
2020-08-28, v3.21.0-rc1~66^2~4).

Help/manual/cmake-generator-expressions.7.rst

@@ -197,6 +197,8 @@ Variable Queries
 
 .. genex:: $<HIP_COMPILER_ID:compiler_ids>
 
+  .. versionadded:: 3.21
+
   where ``compiler_ids`` is a comma-separated list.
   ``1`` if the CMake's compiler id of the HIP compiler matches any one
   of the entries in ``compiler_ids``, otherwise ``0``.
@@ -249,6 +251,8 @@ Variable Queries
 
 .. genex:: $<HIP_COMPILER_VERSION:version>
 
+  .. versionadded:: 3.21
+
   ``1`` if the version of the HIP compiler matches ``version``, otherwise ``0``.
   See also the :variable:`CMAKE_<LANG>_COMPILER_VERSION` variable.
 
@@ -714,6 +718,8 @@ Variable Queries
 
 .. genex:: $<HIP_COMPILER_ID>
 
+  .. versionadded:: 3.21
+
   The CMake's compiler id of the HIP compiler used.
   See also the :variable:`CMAKE_<LANG>_COMPILER_ID` variable.
 
@@ -760,6 +766,8 @@ Variable Queries
 
 .. genex:: $<HIP_COMPILER_VERSION>
 
+  .. versionadded:: 3.21
+
   The version of the HIP compiler used.
   See also the :variable:`CMAKE_<LANG>_COMPILER_VERSION` variable.