This is unfortunately all too common. I am often dinged by my manager about my throughput when investigating the integration of new libraries because I read the docs, front-to-back, as part of my research and that takes time.
I don't think it's really appropriate or feasible or even helpful for most things. At least for myself, if I read through the entirety of a language's documentation I wouldn't remember half of it, and I probably wouldn't need everything I remembered.
Language and library designs should optimize for least-surprise.
Most libraries are quite poorly documented (to my taste, anyhow). It is certainly worth spending the time reading at the very least documentation pertinent to the components of the library intended for use.
