Document static vendor_projects options flags

I was having trouble getting a static vendor project to compile with ARC and found some undocumented options in the source code that would allow me to pass custom CFLAGS. I removed the unused `:headers_dir` flag and added documentation for `:source_files` and `:cflags`. I tried to match the existing wording as closely as possible, but feel free to make any copy changes you want for readability.