Creating a PDF manual for an installed package

I am trying to create a PDF manual for a number of installed and loaded non-CRAN packages, from within RStudio.

library(devtools)
devtools::build_manual(fable, path = "./")

I have tried many variations on this. The most common error I get is

Error: path must be a string.

The first argument must be a path or a "package object". I can not find any definition of a package object. I have found no instance of a string or expression that does not yield some sort of error in

as.package(expr)

Solution

Using the path to the package as the first argument in build_manual() works for me:

sessioninfo::package_info("rlang")
#>  package * version    date       lib source                      
#>  rlang     0.4.0.9000 2019-07-10 [1] Github (r-lib/rlang@a3d7e47)
#> 
devtools::build_manual("~/R/win-library/3.6/rlang", path = ".")
#> Hmm ... looks like a package
#> Creating pdf output from LaTeX ...
#> Saving output to './rlang_0.4.0.9000.pdf' ...
#> Done