[clang] Add clang/docs/FunctionEffectAnalysis.rst. (PR #109855)

[via cfe-commits]

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+Function Effect Analysis
+========================
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+Introduction
+============
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+Clang Function Effect Analysis is a C++ language extension which can warn about "unsafe"
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Sirraide wrote:

I don’t think we need to explain the different attribute syntaxes in too much detail here (you mention them below, and what’s there is enough imo), but that should just say ‘language extension’ w/o the ‘C++’, yeah.

https://github.com/llvm/llvm-project/pull/109855