Update docs

Author: petk

docs/cmake-modules/PHP/SAPIs.md

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+# PHP/SAPIs
+
+See: [SAPIs.cmake](https://github.com/petk/php-build-system/tree/master/cmake/cmake/modules/PHP/SAPIs.cmake)
+
+Add subdirectories of PHP SAPIs via `add_subdirectory()`.
+
+This module is responsible for traversing `CMakeLists.txt` files of PHP SAPIs
+and adding them via `add_subdirectory()`.
+
+## Exposed macro
+
+```cmake
+php_sapis_add(subdirectory)
+```
+
+## Custom CMake properties
+
+* `PHP_ALL_SAPIS`
+
+  Global property with a list of all PHP SAPIs in the sapi directory.
+
+* `PHP_SAPIS`
+
+  This global property contains a list of all enabled PHP SAPIs for the current
+  configuration.