Update docs

Author: petk

docs/cmake/modules/PHP/InterproceduralOptimization.md

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+<!-- This is auto-generated file. -->
+* Source code: [cmake/modules/PHP/Optimization.cmake](https://github.com/petk/php-build-system/blob/master/cmake/cmake/modules/PHP/Optimization.cmake)
+
+# PHP/Optimization
+
+Enable interprocedural optimization (IPO/LTO) on all targets, if supported.
+
+This adds linker flag `-flto` if it is supported by the compiler to run standard
+link-time optimizer.
+
+It can be also controlled more granular with the
+`CMAKE_INTERPROCEDURAL_OPTIMIZATION_<CONFIG>` variables based on the build type.
+
+This module also checks whether IPO/LTO can be enabled based on the PHP
+configuration (due to global register variables) and compiler/platform.
+
+https://cmake.org/cmake/help/latest/prop_tgt/INTERPROCEDURAL_OPTIMIZATION.html
+
+## Basic usage
+
+```cmake
+# CMakeLists.txt
+include(PHP/Optimization)
+```