[clang] [clang][ssaf][docs] Document the Summary Extraction pipeline (PR #172876)

[Balázs Benics via cfe-commits]

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+==================
+Summary Extraction
+==================
+
+.. WARNING:: The framework is rapidly evolving.
+  The documentation might be out-of-sync of the implementation.
+  The purpose of this documentation to give context for upcoming reviews.
+
+
+The simplest way to think about the lifetime of a summary extraction is by following the handlers of the ``FrontendAction`` implementing it.
+There are 3 APIs that are important for us, that are invoked in this order:
+
+  - ``BeingInvocation()``: Checks the command-line arguments related to summary extraction.
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steakhal wrote:

Fixed in 21caf796ff8aeb250ebe603ea9c96ba031b522cfs

https://github.com/llvm/llvm-project/pull/172876