If you’re coming to “ Billie Eilish: The World’s a Little Blurry ” hoping for a primer on the music sensation, you’ve come to the wrong place. Filmmaker R.J. Cutler’s two hour and 20-minute documentary about the “Ocean Eyes” singer and songwriter is not biography or reportage. It’s a verite-style plunge into her life, her home, her concerts, her process, her Tourette’s, her brother’s bedroom where they famously write all their songs and even her diary in the year in which she became a star.