Add a note about SafeHtml.fromConstant.
Add a note about SafeHtml.fromConstant.

RELNOTES: None.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=199329696

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