DOC: coding-style: update a few rules about pointers

DOC: coding-style: update a few rules about pointers

It's really annoying to see that in 2020 we're still facing bugs caused
by dangling pointers in the code that result from poorly written rules
about how these pointers are supposed to be handled, set and reset. Let's
add a few supposedly obvious (but apparently not) rules about how pointers
have to be used through out the code in hope to make such bad practices
disappear (or at least have something to point the authors to after
reviewing their code).