docs: add config info for match_arm_leading_pipes
docs: add config info for match_arm_leading_pipes

aidancully/rust,graydon/rust,rust-lang/rustfmt,aidancully/rust,graydon/rust,SingingTree/rustfmt,rust-lang/rustfmt,graydon/rust,graydon/rust,SingingTree/rustfmt,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust