doc: improve documentation of Bed's API
Fasta: improve and document API

The improvements are just making sure that error types in Result.t are
“open”.
doc.{biohtml,CSS}: add highlighting for elements
Fastq: fix API documentation
Gff: improve documentation
Jaspar: add documentation
Lines: improve documentation
Sam: improve and document API
Wig: open error types
Track: document and improve API
Wig: improve documentation
Zip: improve API documentation
Tags: improve documentation