Add examples of non-get_ getters from standard library to the C-GETTER guideline.

[ely-uf]

The beginning of the C-GETTER guideline states:

With a few exceptions, the get_ prefix is not used for getters in Rust code.

However, all the guideline examples from the standard library have the get_ prefix:

std::io::Cursor::get_mut
std::ptr::Unique::get_mut
std::sync::PoisonError::get_mut
std::sync::atomic::AtomicBool::get_mut
std::collections::hash_map::OccupiedEntry::get_mut
<[T]>::get_unchecked

representing only the following rule of the guideline:

The get naming is used only when there is a single and obvious thing that could reasonably be gotten by a getter. For example Cell::get accesses the content of a Cell.

Suggested examples of non-get_ getters from the standard library:

• std::vec::Vec::len
• std::thread::JoinHandle::thread
• std::panic::Location::file

[KodrAus]

I think the distinction there is between getters as accessors for one of many fields such as MyStruct::get_field1_mut and a single getter for the inner value such as Cursor::get_mut. In the former case, we'd prefer MyStruct::field1_mut.