[vcpkg] Update vcpkg_check_features document (#7091)

* [oniguruma] Fix misusage of vcpkg_check_features

* [xsimd] Use vcpkg_check_features

1 files changed, 39 insertions, 9 deletions

diff --git a/scripts/cmake/vcpkg_check_features.cmake b/scripts/cmake/vcpkg_check_features.cmake
index c8848e987..115b1501f 100644
--- a/scripts/cmake/vcpkg_check_features.cmake
+++ b/scripts/cmake/vcpkg_check_features.cmake
@@ -1,6 +1,6 @@
 ## # vcpkg_check_features
 ##
-## Check if one or more features are part of the package installation. 
+## Check if one or more features are a part of the package installation.
 ##
 ## ## Usage
 ## ```cmake
@@ -11,12 +11,9 @@
 ## )
 ## ```
 ##
-## `vcpkg_check_features` accepts a list of (feature, output_variable) pairs.
-## The syntax is similar to the `PROPERTIES` argument of `set_target_properties`.
+## `vcpkg_check_features` accepts a list of (feature, output_variable) pairs. If a feature is specified, the corresponding output variable will be set as `ON`, or `OFF` otherwise. The syntax is similar to the `PROPERTIES` argument of `set_target_properties`.
 ##
-## `vcpkg_check_features` will create a variable `FEATURE_OPTIONS` in the
-## parent scope, which you can pass as a part of `OPTIONS` argument when
-## calling functions like `vcpkg_config_cmake`:
+## `vcpkg_check_features` will create a variable `FEATURE_OPTIONS` in the parent scope, which you can pass as a part of `OPTIONS` argument when calling functions like `vcpkg_config_cmake`:
 ## ```cmake
 ## vcpkg_config_cmake(
 ##     SOURCE_PATH ${SOURCE_PATH}
@@ -28,13 +25,46 @@
 ## ```
 ##
 ## ## Notes
-## `vcpkg_check_features` is supposed to be called only once. Otherwise, the
-## `FEATURE_OPTIONS` variable set by a previous call will be overwritten.
+## ```cmake
+## vcpkg_check_features(<feature> <output_variable>)
+## ```
+## can be used as a replacement of:
+## ```cmake
+## if(<feature> IN_LIST FEATURES)
+##     set(<output_variable> ON)
+## else()
+##     set(<output_variable> OFF)
+## endif()
+## ```
+##
+## However, if you have a feature that was checked like this before:
+## ```cmake
+## if(<feature> IN_LIST FEATURES)
+##     set(<output_variable> OFF)
+## else()
+##     set(<output_variable> ON)
+## endif()
+## ```
+## then you should not use `vcpkg_check_features` instead. [```oniguruma```](https://github.com/microsoft/vcpkg/blob/master/ports/oniguruma/portfile.cmake), for example, has a feature named `non-posix` which is checked with:
+## ```cmake
+## if("non-posix" IN_LIST FEATURES)
+##     set(ENABLE_POSIX_API OFF)
+## else()
+##     set(ENABLE_POSIX_API ON)
+## endif()
+## ```
+## and by replacing these code with:
+## ```cmake
+## vcpkg_check_features(non-posix ENABLE_POSIX_API)
+## ```
+## is totally wrong.
+##
+## `vcpkg_check_features` is supposed to be called only once. Otherwise, the `FEATURE_OPTIONS` variable set by a previous call will be overwritten.
 ##
 ## ## Examples
 ##
 ## * [czmq](https://github.com/microsoft/vcpkg/blob/master/ports/czmq/portfile.cmake)
-## * [oniguruma](https://github.com/microsoft/vcpkg/blob/master/ports/oniguruma/portfile.cmake)
+## * [xsimd](https://github.com/microsoft/vcpkg/blob/master/ports/xsimd/portfile.cmake)
 ## * [xtensor](https://github.com/microsoft/vcpkg/blob/master/ports/xtensor/portfile.cmake)
 function(vcpkg_check_features)
     cmake_parse_arguments(_vcf "" "" "" ${ARGN})