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

[via cfe-commits]

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+========================
+Function Effect Analysis
+========================
+
+.. contents::
+  :depth: 3
+  :local:
+
+
+Introduction
+============
+
+Clang Function Effect Analysis is a language extension which can warn about "unsafe"
+constructs. The feature is currently tailored for the Performance Constraint attributes,
----------------
Sirraide wrote:

```suggestion
constructs. The feature is currently tailored for the Performance Constraint attributes
```

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