Document the `sql_function!` and `no_arg_sql_function!` macros
Document the `sql_function!` and `no_arg_sql_function!` macros

cascalheira/diesel,mcasper/diesel,diesel-rs/diesel,cascalheira/diesel,sgrif/diesel,SoftwareWarlock/diesel,SoftwareWarlock/diesel,tohou/diesel,sgrif/diesel,diesel-rs/diesel,tohou/diesel,mcasper/diesel