Part of rust-lang#29368.
* added missing links
* updated method summaries to use 3rd person style
* added missing periods in `Component`'s variant summaries
* use standard iterator boilerplate in `Components`' and `Iter`'s docs
* added example to `Iter::as_path`, adapted from `Components::as_path`'s
  example
* consolidated examples for `Path::file_name`
* some other small fixes

Part of rust-lang#29368.

Part of rust-lang#29368.
* Added a new summary paragraph about std::path's parsing facilities
* Slightly exanded `Component`'s docs
* removed the now redundant section on component types from the module docs
* moved the section on path normalization during parsing to the docs on
  `Path::components`
* Clarified difference between `Prefix` and `PrefixComponent` in their
  respecive summary sentences

Part of rust-lang#29368.
* Added explanation for why the struct exists
* Added link to where it is created
* Added example

* Closed an unclosed paren
* seperator -> separator
* deperator -> separator

These show up in rustdoc so need to be correct.

Prior to this commit, the contents of the Unstable Book were assumed to
be unstable features. This commit moves features into 'language features'
or 'library features' subsections. It also moves the 'linker_flavor'
compiler flag into a new 'Compiler Flags' subsection.

Even though it was helpful, I removed the tidy check that
cross-references the SUMMARY.md links with the Unstable Book directory
contents just because it would be difficult to maintain.

Relevant PR: rust-lang#41142.

I just updated a few links to use https instead of http. 

Thank you! ^ _ ^

…teveklabnik

Add top level sections to the Unstable Book.

Prior to this commit, the contents of the Unstable Book were assumed to
be unstable features. This commit moves features into 'language features'
or 'library features' subsections. It also moves the 'linker_flavor'
compiler flag into a new 'Compiler Flags' subsection.

Even though it was helpful, I removed the tidy check that
cross-references the SUMMARY.md links with the Unstable Book directory
contents just because it would be difficult to maintain.

Relevant PR: rust-lang#41142.

Improve std::path docs

Fixes rust-lang#29368.

This PR contains a pretty significant redistribution of some of the module docs to more appropriate places, as well as general expansion, clarification, and additional examples.

For more details, see the commit descriptions.

r? @steveklabnik

update mdbook

Ship some more improvements.

r? @rust-lang/docs

Fix a few stability attributes

These show up in rustdoc so need to be correct.

remove disclaimer from bootstrap/README.md

…=alexcrichton

Disable git caches again

The appveyor ones aren't working anyway (I need to talk to appveyor about them being corrupt) and the travis cache is taking too long to restore and is being killed, which is sometimes causing bad things to happen because the integrity checking made assumptions that restore was atomic.

I could fix the integrity checks, but we clearly need a different approach if restore is taking too long. Let's disable it all for now.

Updated links to use https

I just updated a few links to use https instead of http.

Thank you! 😄