[analyzer][doc] Add user documenation for taint analysis

Checker alpha.security.taint.TaintPropagation now has user documentation for
taint analysis with an example showing external YAML configuration format.
The format of the taint configuration file is now documented under the user
documentation of Clang SA.

Differential Revision: https://reviews.llvm.org/D113251

GitOrigin-RevId: 4aac00a71db31121d70b140d7367e7f9d9992f66
3 files changed