[doc](limitations.rst): Add MPIX_* as a known limitation

Fixes #53

doc/source/limitations.rst

@@ -28,3 +28,22 @@ Other constants
 ---------------
 
 Since other constants are either for information (type, window and communicator's name, processor name and library version string) or with only a portable subset (datarep of file views), their truncation is less at risk of provoking issues in their uses.
+
+Support of MPIX_ Functions
+==========================
+
+MPIX_ functions represent experimental or extended features that are not part of the
+official MPI standard. These functions are provided by some MPI implementations to explore
+new capabilities or optimizations that may be considered for future inclusion in the MPI
+standard. However, the availability and behavior of MPIX_ functions can vary significantly
+between different MPI implementations, leading to potential portability and compatibility
+issues.
+
+Wi4MPI is designed to provide a high degree of portability and interoperability between
+different MPI implementations, facilitating the development and execution of MPI
+applications across a diverse range of HPC environments. To maintain this level of
+compatibility and to ensure a stable and predictable behavior of MPI applications, **Wi4MPI
+does not fully support MPIX_ functions**; this support is provided on a case-by-case basis.
+At momment, **MPIX_Query_cuda_support** is supported. The support of other functions will
+be evaluated on demand.
+