Enclose code in documentation in backticks
base64: backticks for code in documentation
bigint: backticks for code in documentation
bitv: backticks for code in documentation
auto merge of #9523 : huonw/rust/kud1ing-docs, r=huonw

Collation of @kud1ing's work in #9511, #9512, #9513 and #9518.