Document liballoc::rc

This commit introduces a bunch of documentation, fixes some consistency
issues, and just basically brings Rc<T> and Weak<T> up to snuff.
Update docstrings for UnicodeChar::is_digit => is_numeric rename
rustdoc: inherited vis. for struct variant fields

Teach rustdoc that struct variant fields have inherited visibility.

Fix #19048
Documentation fix for std::num::strconv

- int_to_str_bytes_common() doesn't have a return value.
- float_to_str_bytes_common() has an old-style doc comment.
Fixed iOS build after Iter stab
auto merge of #19348 : SimonSapin/rust/patch-9, r=huonw
auto merge of #19112 : steveklabnik/rust/doc_rc, r=Gankro
auto merge of #19349 : tomjakubowski/rust/rustdoc-struct-variant-vis, r=sfackler

Teach rustdoc that struct variant fields have inherited visibility.

Fix #19048
auto merge of #19354 : barosl/rust/strconv-doc-fix, r=steveklabnik

- `int_to_str_bytes_common()` doesn't have a return value.
- `float_to_str_bytes_common()` has an old-style doc comment.
auto merge of #19355 : vhbit/rust/ios-backtrace-fix, r=alexcrichton
Auto-merging dbbb722

Travis: Passed at 2014-11-28T09:33:57Z
https://travis-ci.org/vhbit/rust/builds/42370581