API Docs: ptr

[steveklabnik]

Part of #29329

http://doc.rust-lang.org/std/ptr/

Here's what needs to be done to close out this issue:

• the module docs act like this is about *const T/*mut T, but it's really about common functions that work with them.
• copy could link to memmove somewhere.
• copy_nonoverlapping should link to memcpy. Both of these functions should talk more about what "overlapping" means here.
• drop_in_place needs links and examples
• read needs to explain why and where you should use it, as well as linking some stuff.
• read_volitile needs links
• replace should link to mem::replace, and justify its unsafety better.
• swap needs to split out its summary from its description, link to mem::swap, and get an example.
• write needs to explain when and where to use it, and to link some types
• write_bytes needs to better justify itself, link memset, and move that out of its summary line.
• write_volatile needs to link read.

[steveklabnik]

I am happy to mentor anyone who wants to tackle this issue.