Do not recommend system package usage

Signed-off-by: Uilian Ries <uilianries@gmail.com>

Author: uilianries

reference/tools/gnu/pkgconfig.rst

@@ -29,7 +29,7 @@ Using PkgConfig to fill a ``cpp_info`` object
 ---------------------------------------------
 
 The ``PkgConfig`` class can be used to fill a ``CppInfo`` object with the information that will be consumed by the ``PkgConfigDeps`` generator later.
-This is a useful feature when packaging a system library that provides a ``.pc`` file, or when a proprietary package has a build system that only outputs ``.pc`` files.
+This is a useful feature when packaging a proprietary package has a build system that only outputs ``.pc`` files for a known environment.
 
 .. code-block:: python
 
@@ -78,6 +78,11 @@ The ``.pc`` information will be extracted from the ``cpp_info.json`` file locate
 This way, the ``PkgConfig`` will not need to invoke the ``pkg-config`` executable again to extract the information from the ``.pc`` file,
 when consuming the package.
 
+.. warning::
+
+    It's not recommended to use this approach when creating a "system" package recipe, as the packaged information may not be compatible with the host system,
+    resulting in errors when consuming the package.
+
 Reference
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