#12716: reorganize docs for os module a bit

* Move the constants used by os.open() to after the description of os.open(),
  instead of putting them in their own section.

* Move RTLD_* constants to "Miscellaneous System Information" section, and
  mention the sys functions they're used with.

* Mention dir_fd parameter in os.open() text.

* Typo fix ("are a supported by").