7113: Manual updates r=matklad a=tekul

Add some details on how to build the manual and some clarification on how to deal with "proc macro2 warnings.

For context, this arose from [this question](https://users.rust-lang.org/t/how-to-disable-rust-analyzer-proc-macro-warnings-in-neovim/53150) on users.rust-lang.org.

Co-authored-by: Luke Taylor <tekul.hs@gmail.com>