Add a doc for translating uniform block to StructuredBuffer This document shows when and how shader code would hit the optimization that translates the uniform block into StructuredBuffer to resolve the slow fxc compiling performace with dynamic uniform indexing. Bug: angleproject:3682 Change-Id: I89d94a7e9d812318b9a6cbc470d39ed5ce3e47a0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2561426 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jiajia Qin <jiajia.qin@intel.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>