Document preference of type assertions to casts in goog.asserts
Document preference of type assertions to casts in goog.asserts

RELNOTES: documenting goog.asserts to be a preferred method of type narrowing for the compiler, over jsdoc cast.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211923506

google/closure-library,lucidsoftware/closure-library,teppeis/closure-library,google/closure-library,google/closure-library,google/closure-library,google/closure-library,lucidsoftware/closure-library,teppeis/closure-library,lucidsoftware/closure-library,teppeis/closure-library,teppeis/closure-library,lucidsoftware/closure-library