PW32 comes with no generic POSIX API / ANSI C library documentation. This is done intentionally, I don't want to duplicate or reminisce existing documentation. Instead, I try to be conforming to standards and provide links to existing documentation.
Each (almost) GNU package contains documentation, mostly in TexInfo format, off which GNU info files is generated. Some packages also have Unix man pages. PW32 packages contain all these, but PW32 currently has no GNU info/man ports. So, you may: