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

Follow-on from #99656, which introduces 2nd pass caller/callee analysis
for function effects.

Wrote a new documentation page, derived directly from the RFC posted to
LLVM Discourse earlier this year.

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Co-authored-by: Doug Wyatt <[email protected]>
Co-authored-by: Sirraide <[email protected]>

Author: 3 people