clarify docs for std:io::fs::Path::{is_dir,is_file,exists}; add lstat

Clarifies the interaction of `is_dir`, `is_file` and `exists` with
symbolic links.  Adds a convenience `lstat` function alongside of
`stat`.  Removes references to conditions.

Closes issue #12583.
auto merge of #13750 : aturon/rust/issue-12583, r=alexcrichton

Clarifies the interaction of `is_dir`, `is_file` and `exists` with
symbolic links.  Adds a convenience `lstat` function alongside of
`stat`.  Removes references to conditions.

Closes issue #12583.