Document attributes cfg family

[pnkfelix]

Sub-bug of #6444.

• cfg
  • cfg(debug)
  • cfg(target_arch)
  • cfg(target_endian)
  • cfg(target_family)
  • cfg(target_libc)
  • cfg(target_os): src/librustc/driver/driver.rs
    • "win32","macos","linux","android","freebsd"
  • cfg(target_word_size)
  • cfg(test)

[sanxiyn]

Triaging. We have ndebug instead of debug now, and I couldn't find any evidence of target_libc support. Tagging A-docs.

[alexcrichton]

Assigning as E-easy and E-mentor (I'll mentor this).