update docs (#3372)

reference/conanfile/tools/apple.rst

@@ -83,7 +83,7 @@ case). The above commands generate the following files:
 The first ``conan install`` with the default *Release* and *x86_64* configuration generates: 
 
 - *conan_libpng_libpng_release_x86_64.xcconfig*: declares variables with conditional logic to be considered only for the active configuration in *Xcode* or the one passed by command line to *xcodebuild*.
-- *conan_libpng_libpng.xcconfig*: includes *conan_libpng_libpng_release_x86_64.xcconfig* and declares the following *Xcode* build settings: ``HEADER_SEARCH_PATHS``, ``GCC_PREPROCESSOR_DEFINITIONS``, ``OTHER_CFLAGS``, ``OTHER_CPLUSPLUSFLAGS``, ``FRAMEWORK_SEARCH_PATHS``, ``LIBRARY_SEARCH_PATHS``, ``OTHER_LDFLAGS``. It also includes the generated *xcconfig* files for transitive dependencies (*conan_zlib_zlib.xcconfig* in this case).
+- *conan_libpng_libpng.xcconfig*: includes *conan_libpng_libpng_release_x86_64.xcconfig* and declares the following *Xcode* build settings: ``SYSTEM_HEADER_SEARCH_PATHS``, ``GCC_PREPROCESSOR_DEFINITIONS``, ``OTHER_CFLAGS``, ``OTHER_CPLUSPLUSFLAGS``, ``FRAMEWORK_SEARCH_PATHS``, ``LIBRARY_SEARCH_PATHS``, ``OTHER_LDFLAGS``. It also includes the generated *xcconfig* files for transitive dependencies (*conan_zlib_zlib.xcconfig* in this case).
 - *conan_libpng.xcconfig*: in this case it only includes *conan_libpng_libpng.xcconfig*, but in the case that the required package has components, this file will include all of the components of the package.
 - Same 3 files will be generated for each dependency in the graph. In this case, as *zlib* is a dependency of *libpng* it will generate: *conan_zlib_zlib_release_x86_64.xcconfig*, *conan_zlib_zlib.xcconfig* and *conan_zlib.xcconfig*.
 - *conandeps.xcconfig*: configuration files including all direct dependencies, in this case, it just includes ``conan_libpng.xcconfig``.